Theme overrides stopped working once ClrScheme::maClrScheme was
changed to vector, and colors were always appended to it.

Regression from f3121049.

Change-Id: Iae850dcabf57b12d8a564e84acf38d9988cfe963
Reviewed-on: https://gerrit.libreoffice.org/44242Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>