Dark theme avatars should have brighter background color for transparent images

[bwindels]

As most transparent avatars seem to assume a light background.

[ara4n]

i think this is incorrect, as per #8425.

tl;dr:

• if you want your avatar to have a light background, you should upload one with a light background.
• if you want your avatar to have a dark background, you should upload one with a dark background.
• if you want your avatar to have a transparent background so that it fits into both light and dark themes, you need to design the artwork to work on both light and dark backgrounds (e.g. not upload a black-on-transparent avatar and then wonder why it's invisible in a dark theme).

[rxl881]

I think that @matthew is right, but there will be a lot of existing avatars out there that will look pretty broken / invisible in the current setup. Could this be fixed by adding a hairline border (with the same colour as the room list separators) around the avatars, so that at least you can see that they are there, regardless of the content?