Revert "Move terminal color methods to a new class TerminalColor."

This reverts commit 72d604c6.

BUG: https://bugs.kde.org/show_bug.cgi?id=437223 Can't OPT-OUT of invisible scrollbar slider

Oxygen before:

Oxygen after:

Windows 9x before:

Windows 9x after:

There are no visual differences for Breeze or Fusion.

It also solves visual glitches after using the scrollbar, which seem to be the glitches seen by Grósz Dániel at https://bugs.kde.org/show_bug.cgi?id=393423#c17