Theming System Improvements

Misc Improvements:

• Certain UI elements utilize seemingly random theme attributes (e.g. New Tab uses Graph View theming attributes) This should probably be cleaned up.
• Allow colors passed as [r, g, b] to have a 4th optional alpha field ([r, g, b, a])
• Documentation for stylesheet support and how it interfaces with binaryninja is badly needed.
• Documentation for theming in general needs some improvements.

Feature Suggestions

Theme Inheritance

One critical pain at the moment is that there are no "defaults", or simple method to grab one of the embedded themes as a "template".

Rather than manually exporting the embedded themes and uploading them somewhere, I'd propose a "parent" optional flag in the bntheme that allows inheriting another theme's dictionaries.

This would allow making "theme packs" a bit easier as well

Pallete Brush feature

Currently, themes can define args that specify the palette color, but not the palette brush. This would allow for the ability to load QPixmaps (passed as image urls) as pane backdrops.

Icon Overrides

Not quite sure about this one, but I think it would be a nice feature to have. An access method in the API that fetches the theme override or default would be great.

Start by mapping the existing stylesheet support, bntheme dictionaries, embedded themes, palette color parsing, and theme override/default API. Review how the proposed parent flag, alpha values, palette brushes, and icon overrides relate to those entry points. Done would require a defined scope for the selected improvements plus documentation for stylesheet support and theming.