diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..1250f57
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,72 @@
+# Contributing
+
+## Adding a New Theme
+
+To add your theme, copy your CSS file into the `themes` directory. The name of your file has to match the format `prism-{your-theme-id}.css`. Make sure your theme follows your guidelines.
+
+Next, you have to register your theme in `themes.json`. Add a new entry like this:
+
+```json
+"{your-theme-id}": {
+    "title": "Your title",
+    "owner": "Your GitHub user name"
+}
+```
+
+Additionally, you can also specify who the original owner is (`originalOwner`) or on what theme your theme is based (`basedOn`).
+You aren't required to give a GitHub user name. You can also use a different name and link where the link is optional:
+
+```json
+"{your-theme-id}": {
+    "title": "Your title",
+    "owner": {
+        "name": "My real name",
+        "link": "https://github.com/someUserName"
+    },
+    "originalOwner": {
+        "name": "Famous artist"
+    }
+}
+```
+
+At last, you have to rebuild the project by running:
+
+```bash
+npm i && npm run build
+```
+
+Before making a pull request, you can use the following command to verify that all our checks pass and that your theme follows all our guidelines:
+
+```bash
+npm test
+```
+
+Thank you so much for contributing!!
+
+
+## Theme guidelines and requirements
+
+Your theme will be used by Prism and its plugins which can also add CSS rules.
+These guidelines will make sure that your theme is compatible with Prism and its plugins.
+
+### Font size of code blocks
+
+Some of Prism's plugins assume that the `pre` and `code` elements have the same font size.
+
+To make sure that this is that case, all themes are __required__ to either include a rule which ensures that the property `font-size: 1em` is applied to all elements which match `pre > code[class*="language-"]` or to not change the font size of code blocks.
+
+If you want to change the font size of code blocks, use the `font-size` property on the `pre` element. Example:
+
+```css
+pre[class*="language-"] {
+    font-size: 90%;
+}
+```
+
+### Overflow of `pre` elements
+
+The `overflow` property of `pre` elements is __required__ to be left unset or set to `auto`.
+
+This is required because other `overflow` values will cause problems with layouts based on `float` or flexboxes.
+
+One notable exception is the original Coy theme because its shadows are impossible to implement otherwise.
diff --git a/README.md b/README.md
index 9ca5c0b..e892095 100644
--- a/README.md
+++ b/README.md
@@ -22,22 +22,7 @@ To use one of the themes, just include the theme's CSS file in your page. Exampl
 
 ## Adding a New Theme
 
-To add your own theme, copy it into the `themes` directory and add your themes to the list of available themes in the readme.
-The links for your themes have to be `themes/prism-<your theme>.css` for the theme itself and `screenshots/prism-<your theme>.png` for the screenshot.
-
-The screenshot will be created for you by running the following command:
-
-```bash
-npm i && npx gulp screenshot
-```
-
-Before making a pull request, you can the following command to verify that all checks pass:
-
-```bash
-npm test
-```
-
-Thank you so much for contributing!!
+See our [Contributing](CONTRIBUTING.md) guide.
 
 ## Available themes
 
@@ -54,7 +39,7 @@ Thank you so much for contributing!!
 [![Xonokai](screenshots/prism-xonokai.png)](themes/prism-xonokai.css)
 
 * [__Ateliersulphurpool-light__](themes/prism-base16-ateliersulphurpool.light.css) (by [Bram de Haan](https://github.com/atelierbram))<br />
-[![Ateliersulpherpool-light](screenshots/prism-base16-ateliersulphurpool.light.png)](themes/prism-base16-ateliersulphurpool.light.css)
+[![Ateliersulphurpool-light](screenshots/prism-base16-ateliersulphurpool.light.png)](themes/prism-base16-ateliersulphurpool.light.css)
 
 * [__Hopscotch__](themes/prism-hopscotch.css) (by [Jan T. Sott](https://github.com/idleberg))<br />
 [![Hopscotch](screenshots/prism-hopscotch.png)](themes/prism-hopscotch.css)
@@ -83,14 +68,14 @@ Thank you so much for contributing!!
 * [__VS__](themes/prism-vs.css) (by [andrewlock](https://github.com/andrewlock))<br />
 [![VS](screenshots/prism-vs.png)](themes/prism-vs.css)
 
-* [__Darcula__](themes/prism-darcula.css) (by [service-paradis](https://github.com/service-paradis), based on Jetbrains Darcula theme)<br />
+* [__Darcula__](themes/prism-darcula.css) (by [service-paradis](https://github.com/service-paradis), based on Jetbrain's Darcula theme)<br />
 [![Darcula](screenshots/prism-darcula.png)](themes/prism-darcula.css)
 
 * [__a11y Dark__](themes/prism-a11y-dark.css) (by [ericwbailey](https://github.com/ericwbailey))<br />
 [![a11y Dark](screenshots/prism-a11y-dark.png)](themes/prism-a11y-dark.css)
 
 * [__Dracula__](themes/prism-dracula.css) (by [Byverdu](https://github.com/byverdu))<br />
-[![a11y Dark](screenshots/prism-dracula.png)](themes/prism-dracula.css)
+[![Dracula](screenshots/prism-dracula.png)](themes/prism-dracula.css)
 
 * [__Synthwave '84__](themes/prism-synthwave84.css) (originally by [Robb Owen](https://github.com/robb0wen), adapted by [Marc Backes](https://github.com/themarcba))<br />
 [![Synthwave '84](screenshots/prism-synthwave84.png)](themes/prism-synthwave84.css)
diff --git a/gulpfile.js b/gulpfile.js
index 2f5a680..5f9ed72 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -2,16 +2,33 @@ const fs = require('fs').promises;
 const { parallel } = require('gulp');
 const captureWebsite = require('capture-website');
 const path = require('path');
+const { JSDOM } = require('jsdom');
+
+
+/**
+ * @type {Object<string, ThemesEntry>}
+ *
+ * @typedef ThemesEntry
+ * @property {string} title
+ * @property {Entity} owner
+ * @property {Entity} [originalOwner]
+ * @property {Entity} [basedOn]
+ *
+ * @typedef {string | { name: string; link?: string }} Entity
+ */
+const themeCatalog = require('./themes.json').themes;
 
 
 const themesDir = path.join(__dirname, 'themes');
 const screenshotDir = path.join(__dirname, 'screenshots');
+const getThemePath = theme => path.join(themesDir, `prism-${theme}.css`);
+const getScreenshotPath = theme => path.join(screenshotDir, `prism-${theme}.png`);
 
 /**
  * Returns the names of all themes. This includes the `prism-` prefix.
  */
 async function getThemes() {
-	return (await fs.readdir(themesDir)).map(f => (/^.+(?=\.css$)/.exec(f) || [''])[0]).filter(f => f);
+	return Object.keys(themeCatalog);
 }
 
 /**
@@ -41,7 +58,7 @@ async function screenshotMissingThemes() {
  * @param {boolean} overwrite
  */
 async function screenshotTheme(theme, overwrite) {
-	const file = `${screenshotDir}/${theme}.png`;
+	const file = getScreenshotPath(theme);
 
 	if (await fs.stat(file).then(s => s.isFile()).catch(() => false)) {
 		if (overwrite) {
@@ -56,39 +73,183 @@ async function screenshotTheme(theme, overwrite) {
 		scaleFactor: 1,
 		element: 'pre',
 		styles: [
-			await fs.readFile(`${themesDir}/${theme}.css`, 'utf-8')
+			await fs.readFile(getThemePath(theme), 'utf-8')
 		]
 	});
 }
 
+/**
+ * Updates the "Available themes" section in `README.md`.
+ */
+async function updateReadme() {
+	const themes = await getThemes();
+
+	/**
+	 * Returns the credit string of a theme.
+	 *
+	 * @param {ThemesEntry} entry
+	 * @returns {string}
+	 */
+	function getCredit(entry) {
+		if (entry.basedOn) {
+			return `by ${printEntity(entry.owner)}, based on ${printEntity(entry.basedOn)}`;
+		} else if (entry.originalOwner) {
+			return `originally by ${printEntity(entry.originalOwner)}, adapted by ${printEntity(entry.owner)}`;
+		} else {
+			return `by ${printEntity(entry.owner)}`;
+		}
+	}
+	/**
+	 * @param {Entity} entity
+	 */
+	function printEntity(entity) {
+		if (typeof entity === 'string') {
+			return `[${entity}](https://github.com/${entity})`;
+		} else if (entity.link) {
+			return `[${entity.name}](${entity.link})`;
+		} else {
+			return entity.name;
+		}
+	}
+
+	const md = themes.map(theme => {
+		const css = `themes/prism-${theme}.css`;
+		const screenshot = `screenshots/prism-${theme}.png`;
+
+		const entry = themeCatalog[theme];
+		const title = entry.title;
+		const credit = getCredit(entry);
+
+		return `* [__${title}__](${css}) (${credit})<br />\n[![${title}](${screenshot})](${css})`;
+	}).join('\n\n');
+
+	const readmePath = path.join(__dirname, 'README.md');
+	let readme = await fs.readFile(readmePath, 'utf-8');
+	readme = readme.replace(/(## Available themes)[\s\S]*/, (m, header) => {
+		return `${header}\n\n${md}\n`;
+	});
+
+	await fs.writeFile(readmePath, readme, 'utf-8');
+}
+
 /**
  * Checks that all themes have a screenshot.
  */
 async function checkScreenshots() {
-	for (const theme of await getThemes()) {
-		const file = `${screenshotDir}/${theme}.png`;
-		if (!await fs.stat(file).then(s => s.isFile()).catch(() => false)) {
-			throw new Error(`The theme "${theme}" doesn't have a screenshot.`);
+	const themes = new Set(await getThemes());
+	const screenshots = new Set(await fs.readdir(screenshotDir));
+	screenshots.delete('code.html');
+
+	for (const theme of themes) {
+		if (!screenshots.delete(`prism-${theme}.png`)) {
+			throw new Error(`The theme "${theme}" does not have a screenshot.`);
 		}
 	}
+
+	if (screenshots.size > 0) {
+		throw new Error(`There are screenshots without a theme: "${[...screenshots].join('", "')}"`);
+	}
 }
 
 /**
- * Checks that all themes are in the list of available themes.
+ * Checks that all themes have a CSS file.
  */
-async function checkAvailableThemes() {
-	const readme = await fs.readFile(path.join(__dirname, 'README.md'), 'utf-8');
+async function checkCSS() {
+	const themes = new Set(await getThemes());
+	const cssFiles = new Set(await fs.readdir(themesDir));
 
-	for (const theme of await getThemes()) {
-		if (!readme.includes(theme + ".css")) {
-			throw new Error(`The theme "${theme}" is not included in the list of available themes.`);
+	for (const theme of themes) {
+		if (!cssFiles.delete(`prism-${theme}.css`)) {
+			throw new Error(`The theme "${theme}" does not have a screenshot.`);
 		}
-		if (!readme.includes(theme + ".png")) {
-			throw new Error(`The screenshot of "${theme}" is not included in the list of available themes.`);
+	}
+
+	if (cssFiles.size > 0) {
+		throw new Error(`There are CSS files without a theme: "${[...cssFiles].join('", "')}"`);
+	}
+}
+
+/**
+ * @type {Object<string, Object<string, string | RegExp>>}
+ */
+const requirements = {
+	'pre': {
+		/* Code block: pre */
+
+		'overflow': 'auto'
+	},
+	'pre > code': {
+		/* Code block: code */
+
+		'font-size': /^(?:1em|)$/
+	},
+	':not(pre) > code': {
+		/* Inline: code */
+
+		// none
+	}
+};
+
+async function checkRequirements() {
+	const getSource = css => {
+		return `
+			<!DOCTYPE html>
+			<html>
+			<head>
+				<style>${css}</style>
+			</head>
+			<body>
+				<!-- Code block -->
+				<pre class="language-javascript"><code class="language-javascript">var a = 0;</code></pre>
+
+				<!-- Inline code -->
+				<code class="language-javascript">a++</code>
+			</body>
+			</html>
+		`;
+	};
+
+	let pass = true;
+
+	for (const theme of await getThemes()) {
+		const dom = new JSDOM(getSource(await fs.readFile(getThemePath(theme), 'utf-8')));
+
+		for (const selector in requirements) {
+			const properties = requirements[selector];
+
+			for (const element of dom.window.document.querySelectorAll(selector)) {
+				const style = dom.window.getComputedStyle(element);
+
+				for (const property in properties) {
+					const expected = properties[property];
+					const actual = style[property];
+
+					let valid;
+					if (typeof expected === 'string') {
+						valid = expected === actual;
+					} else {
+						valid = expected.test(actual);
+					}
+
+					if (!valid) {
+						pass = false;
+						console.error(`${theme} does not meet the requirement for "${selector}":\n` +
+							`  Expected the ${property} property to be ${expected} but found "${actual}"`);
+					}
+				}
+			}
 		}
 	}
+
+	if (!pass) {
+		throw new Error('Some checks failed.');
+	}
 }
 
+
+exports['update-readme'] = updateReadme;
 exports.screenshot = screenshotMissingThemes;
 exports['screenshot-all'] = screenshotAllThemes;
-exports.check = parallel(checkScreenshots, checkAvailableThemes)
+exports.build = parallel(screenshotMissingThemes, updateReadme);
+
+exports.check = parallel(checkScreenshots, checkCSS, checkRequirements)
diff --git a/package.json b/package.json
index 25e6879..c591d57 100644
--- a/package.json
+++ b/package.json
@@ -4,6 +4,7 @@
   "description": "Additional themes for the Prism syntax highlighting library.",
   "main": "README.md",
   "scripts": {
+    "build": "npx gulp build",
     "lint": "stylelint \"themes/*.css\"",
     "lint-fix": "stylelint \"themes/*.css\" --fix",
     "test": "npm run lint && gulp check"
@@ -21,6 +22,7 @@
   "devDependencies": {
     "capture-website": "^0.4.0",
     "gulp": "^4.0.2",
+    "jsdom": "^15.2.1",
     "stylelint": "^12.0.0",
     "stylelint-config-standard": "^19.0.0"
   }
diff --git a/themes.json b/themes.json
new file mode 100644
index 0000000..5865b4e
--- /dev/null
+++ b/themes.json
@@ -0,0 +1,172 @@
+{
+	"themes": {
+		"cb": {
+			"title": "CB",
+			"owner": "atelierbram",
+			"originalOwner": {
+				"name": "C. Bavota",
+				"link": "https://bitbucket.org/cbavota"
+			}
+		},
+		"ghcolors": {
+			"title": "GHColors",
+			"owner": "aviaryan"
+		},
+		"pojoaque": {
+			"title": "Pojoaque",
+			"owner": "atelierbram",
+			"originalOwner": {
+				"name": "Jason Tate",
+				"link": "http://web-cms-designs.com/ftopict-10-pojoaque-style-for-highlight-js-code-highlighter.html"
+			}
+		},
+		"xonokai": {
+			"title": "Xonokai",
+			"owner": "atelierbram",
+			"originalOwner": {
+				"name": "Maxime Thirouin (MoOx)",
+				"link": "https://github.com/MoOx"
+			}
+		},
+		"base16-ateliersulphurpool.light": {
+			"title": "Ateliersulphurpool-light",
+			"owner": {
+				"name": "Bram de Haan",
+				"link": "https://github.com/atelierbram"
+			}
+		},
+		"hopscotch": {
+			"title": "Hopscotch",
+			"owner": {
+				"name": "Jan T. Sott",
+				"link": "https://github.com/idleberg"
+			}
+		},
+		"atom-dark": {
+			"title": "Atom Dark",
+			"owner": "gibsjose",
+			"basedOn": {
+				"name": "Atom Dark Syntax theme",
+				"link": "https://github.com/atom/atom-dark-syntax"
+			}
+		},
+		"duotone-dark": {
+			"title": "Duotone Dark",
+			"owner": {
+				"name": "Simurai",
+				"link": "https://github.com/simurai"
+			},
+			"basedOn": {
+				"name": "Duotone Dark Syntax theme for Atom",
+				"link": "https://github.com/simurai/duotone-dark-syntax"
+			}
+		},
+		"duotone-sea": {
+			"title": "Duotone Sea",
+			"owner": {
+				"name": "Simurai",
+				"link": "https://github.com/simurai"
+			},
+			"basedOn": {
+				"name": "DuoTone Dark Sea Syntax theme for Atom",
+				"link": "https://github.com/simurai/duotone-dark-sea-syntax"
+			}
+		},
+		"duotone-space": {
+			"title": "Duotone Space",
+			"owner": {
+				"name": "Simurai",
+				"link": "https://github.com/simurai"
+			},
+			"basedOn": {
+				"name": "DuoTone Dark Space Syntax theme for Atom",
+				"link": "https://github.com/simurai/duotone-dark-space-syntax"
+			}
+		},
+		"duotone-earth": {
+			"title": "Duotone Earth",
+			"owner": {
+				"name": "Simurai",
+				"link": "https://github.com/simurai"
+			},
+			"basedOn": {
+				"name": "DuoTone Dark Earth Syntax theme for Atom",
+				"link": "https://github.com/simurai/duotone-dark-earth-syntax"
+			}
+		},
+		"duotone-forest": {
+			"title": "Duotone Forest",
+			"owner": {
+				"name": "Simurai",
+				"link": "https://github.com/simurai"
+			},
+			"basedOn": {
+				"name": "DuoTone Dark Forest Syntax theme for Atom",
+				"link": "https://github.com/simurai/duotone-dark-forest-syntax"
+			}
+		},
+		"duotone-light": {
+			"title": "Duotone Light",
+			"owner": {
+				"name": "Simurai",
+				"link": "https://github.com/simurai"
+			},
+			"basedOn": {
+				"name": "DuoTone Light Syntax theme",
+				"link": "https://github.com/simurai/duotone-light-syntax"
+			}
+		},
+		"vs": {
+			"title": "VS",
+			"owner": "andrewlock"
+		},
+		"darcula": {
+			"title": "Darcula",
+			"owner": "service-paradis",
+			"basedOn": {
+				"name": "Jetbrain's Darcula theme"
+			}
+		},
+		"a11y-dark": {
+			"title": "a11y Dark",
+			"owner": "ericwbailey"
+		},
+		"dracula": {
+			"title": "Dracula",
+			"owner": {
+				"name": "Byverdu",
+				"link": "https://github.com/byverdu"
+			}
+		},
+		"synthwave84": {
+			"title": "Synthwave '84",
+			"owner": {
+				"name": "Marc Backes",
+				"link": "https://github.com/themarcba"
+			},
+			"originalOwner": {
+				"name": "Robb Owen",
+				"link": "https://github.com/robb0wen"
+			}
+		},
+		"shades-of-purple": {
+			"title": "Shades of Purple",
+			"owner": {
+				"name": "Ahmad Awais",
+				"link": "https://github.com/ahmadawais"
+			}
+		},
+		"material-dark": {
+			"title": "Material Dark",
+			"owner": "dutchenkoOleg"
+		},
+		"material-light": {
+			"title": "Material Light",
+			"owner": "dutchenkoOleg"
+		},
+		"material-oceanic": {
+			"title": "Material Oceanic",
+			"owner": "dutchenkoOleg"
+		}
+	}
+}
diff --git a/themes/prism-shades-of-purple.css b/themes/prism-shades-of-purple.css
index a4a0cd5..76c45bd 100644
--- a/themes/prism-shades-of-purple.css
+++ b/themes/prism-shades-of-purple.css
@@ -43,6 +43,10 @@ code[class*='language-'] ::selection {
 	background: #a599e9;
 }
 
+pre > code[class*="language-"] {
+	font-size: 1em;
+}
+
 /* Code blocks. */
 pre[class*='language-'] {
 	padding: 2em;