Support opacity in selectionBorder theme color

I use a tiny 6x10 font similar to Proggy that makes use of the top row of pixels in each glyph. The top row gets concealed whenever there's a selection, and I'd like to make the selectionBorder color partly transparent to avoid obscuring important parts of some characters (e.g. the top of an 'f'), e.g. by changing the color in the theme from #222218 to #22221880 or something. This doesn't seem to work yet on Windows.