Allow color customization of single togglable

[Jupi007]

For example:

This is the part one of yaru togglable theming.
In a second time, maybe introduce some kind of app global theming? Wdyt @jpnurmi @Feichtmeier ?

Fixes #526

Pull request checklist

• This PR does not introduce visual changes

[Feichtmeier]

nice

[jpnurmi]

It would be nice to have:

final MaterialStateProperty<Color?>? checkColor;
final MaterialStateProperty<Color?>? fillColor;

and then introduce YaruCheckboxTheme(Data) with the same properties.

[jpnurmi]

See also:

• https://api.flutter.dev/flutter/material/CheckboxThemeData/checkColor.html
• https://api.flutter.dev/flutter/material/CheckboxThemeData/fillColor.html

[Jupi007]

Yeah, I looked at how material widgets are done.
But isn't ask for a MaterialStateProperty a bit overkill? Because we don't provide hover/focus variants.

[jpnurmi]

or maybe 🤔

class YaruCheckbox {
  final Color? checkColor;
  final Color? fillColor;
}

class YaruCheckboxThemeData {
  final MaterialStateProperty<Color?>? checkColor;
  final MaterialStateProperty<Color?>? fillColor;
}

[jpnurmi]

It's up to us decide and document which states each widget resolves against. :) As for togglables, selected and disabled states at least?

[Jupi007]

It's up to us decide and document which states each widget resolves against. :) As for togglables, selected and disabled states at least?

Okay, I didn't know :D
Let's do this so 👍