Theme system

[htotoo]

This will add whole system theme options.
While checking, please look for missing or unreadable texts, and report them with screenshots.

Also in the future apps it will be required (if merged) to use colors + styles from the Theme class.

[htotoo]

The important part is theme.hpp, theme.cpp.
Oc the others are important too, for example the default theme should match the original. Like color::grey() ->Theme::... fg_medium.foreground or bg_medium.background

If I swapped it somewhere there could be missing or unreadable text. (Like black on black, ...)

[gullradriel]

Looks good to me, let's merge and ask people to test and report if you missed some widgets, else it's an awesome work.
Good job, thanks :-)

[gullradriel]

After a bit of testing everything looks good to me, at least enough for merging.