Don't configure colors

[jvolante]

I'd like to be able to use Stylix to auto configure fonts, cursors and wallpapers, but I don't want it to handle coloring because I don't like the way it applies colors to many applications. Is this possible under the current setup?

[trueNAHO]

Related: https://github.com/danth/stylix/issues/216

[danth]

For each of the items Stylix themes:

• Colors
• Wallpaper
• Fonts

We should support a value of null which would disable all theming for that element. This could be a possible alternative to #216.

Also related: #200

[Lyndeno]

For those three elements, I think it would be good to have an enable option per-target.

E.g. I don't want to theme Gnome's colours, but I still want font theming, and colours for my other applications.

[danielgafni]

In general, applying stylix should be more versatile. I think it would be very useful if:

• it was possible to disable some of (fonts, colors, ...)
• it was possible to disable some of (fonts, colors, ...) for specific targets
• it was possible to override any value for specific targets