Accessibility: Themes with higher contrast

[Ivaryna]

Both the light and the dark theme have rather low contrast, making it hard to read for some users. Adding high contrast versions or allowing the users to chose their own colors would alleviate this problem

[per1234]

or allowing the users to chose their own colors would alleviate this problem

This part of the feature request was previously suggested in #6454
but the "add high contrast themes" is original.