You might be able to use "fill: white" or whatever color, although I'm not sure if that works with <img>'d SVGs or just just inline ones.
That wouldn't work in this case. You can only style inlined svgs, and these are svg as img tags.
Ambious' drop shadow idea is interesting, though.
A plainer option is to use filter: invert(100%)
, which works great on monochrome, less great on the colored icons.