Make Compatible with dark colour schemes and palettes

[MartinF99]

Currently Plastikstyle uses lots of fixed .darker() and .lighter() calls for highlights borders and so on, Either we can make an inverted Plastik-dark for dark colourschemes or replace these calls with something better.

[hazel-bunny]

Plastik-dark would be better if hardcoded colors aren't used, as it will retain most of the original plastik code

[MartinF99]

the issue is that Plastik typically calls color.lighter(val) and color.darker(val). which kinda hardcodes the gradient lightness (which works well for light themes but badly for dark themes)