> But there could be better ways of doing this.
One way would be to define such colors via preferences as the CSS can
style these preferences.
To be honest, I fundamentally think that CSS is a bad answer for such use-case.
In general, any special control is likely to want by default to reuse the parent background and so on, and would be more agile and portable dynamically choosing the best colors according to its container than by assuming a CSS is there to define the right color.
CSS is still a bunch of hardcoded colors. If tomorrow Phaser Editor decides to go for the Hello Kitty theme, then your suggestion about CSS would require Phaser Editor to write specific code in order to work on the light pink background and not be drawn as white.
CSS inheritance could be an intermediary solution here. Is it ignored? Or is it just that Canvas background color is not set in general and then while/default is used?