Improve UI compatbility with dark color schemes

[MadCatX]

Those of us night people who use dark color schemes may find some of the visual elements difficult to read because the colors used by QtMips UI assume a white color scheme. See the following images for some examples:

OK (Core view) - white color scheme OK (Source editor) - white color scheme Not OK (Core view) - Dark color scheme Not OK (Source editor) - Dark color scheme

This small patch series forces the problematic palette colors to shades that are compatible with a white color scheme instead of inheriting them from the system palette.

Fixed (Core view) - Dark color scheme Fixed (Source editor) - Dark color scheme

[ppisa]

Thanks for contribution. I am taking change as it is. I try to think about followup adjustment to not change light (grey, toned) backgrounds to strict white. But I consider to switch to black on white as only feasible solution for dark background and limited time to test and develop QtMips.