Theming support for rat-salsa.

Splits the theme in two parts:

• Scheme - The underlying color-scheme with enough colors to play around.

• One concrete DarkTheme that takes that scheme and produces Styles for widgets. This intentionally doesn't adhere to any trait, just provides some baselines for each widget type. You can use this as is, or copy it and adapt it for your applications needs.

    In the end I think this will be just some building-blocks for 
    an application defined theme. I think most applications will need
    more semantics than just 'some table', 'some list'.