Gregory's Server*** Do not use dark/white transparent logos as avatars, users with the same theme won't be able to see it!
If you want single color logo as avatar, consider removing transparency by having inverted color background (#333 or darker for white logo, #ddd or lighter for dark logo).
@sasha_sorokin I'd love to have a background fallback on avatars but people will probably complain that their cool transparent logos don't look as cool