Apply custom theme to FLTK

Apply custom theme to FLTK

Inspired by modern Windows appearance, and to some extent macOS. They
have flat boxes and use white, or very light, colors for interactive
elements. Unfortunately we can't directly control the colors of
widgets, so instead we just lighten everything that uses this box type.

GNOME uses a different design, both their older and newer style. But UI
look is less consistent on Linux, so hopefully our new look is decent
enough there as well.