Allow themes to modify colors of tabs and option selections

[mantacid]

Currently, the colors for selected items and for frame multiplexer tabs are hard-coded into their respective source files. While one can easily change the colors and rebuild the binary, it would be nice if theme definitions could define these colors to be more visually coherent with the selected theme.

[garlic0x1]

If someone works on this, please do it for the modeline too.

[mantacid]

If someone works on this, please do it for the modeline too.

If you use the modeline and modeline-inactive properties in your theme definition, you already can. They're used in the definitions for the emacs themes in the src/ext/themes.lisp file.

[mantacid]

I will also add that fonts should be easier to select, as the default that comes with lem has some interesting behavior with the lowercase m specifically.