preferences: Don't call changeIconsColors() twice / Restore previous sense of workaround

but instead accumulate a boolean whether to call it once at the end. This will avoid unnecessary work removing/re-adding intermediate CSS.

And a second commit removing some dead code.

And something that seems to help inbox#8863 (comment 1481477715) by restoring the sense of what I think a prior workaround was trying to do, although I'm not sure it still works-around...