Custom color support leads to weird UI rendering

[dnadlinger]

Even without any custom colors set, the custom color support added in 52c07a2 interferes with normal widget styling, causing odd-looking results.

One commit before 52c07a2:

Current master (0f0f030):

This is my local dev box with macOS in particular, but I imagine the same will happen on all sorts of other platforms (e.g. dark mode?). Generally, interfering with widget styling in an "always-on" fashion seems incredibly brittle – sb10q found something similar on his (presumably Linux) box on an earlier version of #2590. Leaving the default styling alone if no colours are set seems like the much better way, unless we want to test all sorts of OS/theming combinations.

[sbourdeauducq]

Note that macOS is not a supported platform.

But it does seem that e.g. the gradients were broken on all platforms by #2590. And the gradients still should be there even with the colors applied.

[dnadlinger]

Yes, I am by no means complaining about macOS specifically – it just supports my broader point that messing with the default OS rendering "without reason" (on the default path of a somewhat niche optional feature) may be more brittle than we should be happy with.

For instance, here is another problem on Windows (the spin box field is grey, almost as if it were disabled):

Just not setting any custom styles at all when no custom colour is configured would avoid issues like this entirely.