avatar: Have a dark-mode style
I tried to use HdyAvatar
as a fallback cover in a dark-mode app, and while it looks nice it's definitely too bright:
![avatar](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
(please note this is a mockup, I know the initials don't match)
We should offer a dark mode style for it, it hurts the eyes otherwise.