Default avatar url and colors should be skinnable

[abeluck]

Since the default avatar urls are images, not css, they cannot be changed by a skin/theme.

ref: https://github.com/matrix-org/matrix-react-sdk/blob/master/src/Avatar.js#L53-L60

In #7569 it was mentioned removing the images and replacing them with background colors. This seems like a good idea. I think that issue got lost/ignored since #8474.

[jryans]

Yes, I agree we should change to CSS for these. It's a bit silly to use images here.

[bl1mp]

indeed

[fooness]

It also would be great to define config.json css variables in light-custom/css/_custom.css to be able to customize the avatar background AND foreground color …

[robintown]

Fixed by matrix-org/matrix-react-sdk#4503