The x11-themes/murrine-themes are old and mate-appearance-properties complains alot when these are installed. Seems to me tha MATE does not need the murrine-themes but do need x11-themes/gtk-engines-murrine.
ping?
Well, it shouldn't rely on murrine-themes but should rely on gtk-engine-murrine, which will pull in the themes unless you disable them via USE flag. I'll update the deps when I can.
Resolved in b62a6fd51e32e7d486620bbf9ac8ed1f273a06a1