Paletize the Cutter color themes

Is your feature request related to a problem? Please describe.

Qt stylesheets support using the colors from current QPallete https://doc.qt.io/qt-5/stylesheet-reference.html#paletterole .

Reasons for using color palette and palette roles in the color theme stylesheets

• less places that need to updated when adjusting color
• might allow simplifying the stylesheets in places where the widgets would use those colors from palette by default
• Dark and midnight theme stylesheets are almost identical except colors. With color palette they can probably be deduplicated thus reducing amount of work for fixing stylesheet related layout bugs.
• easier future theme creation by customizing only palette

Describe the solution you'd like

• Fully specify the color palettes for the existing cutter themes, currently only text color is set
• Replace use of explicit colors in theme stylesheets with palette roles as much as possible
• try if parts of stylesheet can be simplified due to use of palette color being the default behavior
• evaluate if some of the themes can share stylesheet

Describe alternatives you've considered