This allows for color changes and scaling of the arrow
when the display needs it (e.g. HiDPI displays).
Also make the arrow and text respond to if the window is
active to mimic native controls.
Fixes https://gitlab.com/kicad/code/kicad/issues/6042
Script added to generate dark theme from light.
Target colors can be adjusted and then icons can be
re-generated until we're happy with the starting point,
then individual icons can be tweaked.
Ian McInerney
Jon Evans