Support OS accent colors in color schemes

[christianparpart]

MacOS, Windows, as well as KDE (most likely other UNIX DEs) do support accent colors.

It would be nice to make use of that when declaring a colorscheme. This could be mostly usefull for selection foreground/background colors, but maybe also somewhere else.

Accent colors should be mixable with other colors by X% in order to enable variation

Example ideas

default:
    # ...
    background: [accent_mix: '#bfceff', accent_factor: 0.75]
selection:
    foreground: [invert: accent]
    background: accent

[szxnbnb66666]

good