Just in case anyone is wondering, the background color for creating your ribbon icons in the 2015 Dark Scheme is (strangley enough) the same as it is for the light background... (192,192,192).
Hope it helps!
Have you figured out how to get custom icons to change between the dark and light them? The icons I have are using Black Lines - and I want them to still be visible when someone uses the dark theme.
Assuming that you have this answer already, however i'll add my 2 cents.
The CUI now supports .png file types for icons (instead of the age old bitmaps we've been forced to use). PNG files can be exproted with transparent backgrounds making them usable in either light or dark color schemes.