style: adjust dark mode text color contrast

- Increase darkness of $dark-fourth-text-color for better readability
- Change from 60% to 70% darken value for improved contrast in dark mode

source/css/common/colors.styl

@@ -73,7 +73,7 @@ $dark-invert-text-color = #373D3F
 $dark-first-text-color = lighten($dark-default-text-color, 30%)
 $dark-second-text-color = lighten($dark-default-text-color, 20%)
 $dark-third-text-color = darken($dark-default-text-color, 20%)
-$dark-fourth-text-color = darken($dark-default-text-color, 60%)
+$dark-fourth-text-color = darken($dark-default-text-color, 70%)
 $dark-border-color = rgba(255, 255, 255, 0.08)
 $dark-selection-color = $selection-color
 $dark-shadow-color-1 = rgba(255, 255, 255, 0.08)