As the summary says, this patch removes some magic 0, 1, 2 and 3
and uses THEME_* instead. It also removes some magic 0.0, 1.0, 2.0
and 3.0, guessing they are the same thing.

The declaration of THEME_* could use enum or be placed in other
file, however the other part of the patch should work fine anyway.
Remove the FIXME comment once this detail is final (thanks).

I would also remove the condition that OldSkool does not align, as
the layouts do not seem to allow that nicely due to small separations
and in cases maybe random, but that is a different topic (it
would require looking at what supposedly the style follows, Blender
1.x, and check how it really was... other thing that seems out of
place is the rounded corners so the bad look gets even worse...
but as I said, for another day & patch).