Initially implement new empty avatar background

Created by: quinnkeast

This updates the default avatar background from a simple gradient to a slightly more complex 4-colour gradient. This was originally designed as part of the redesign, but we didn't carry it out in full. I'm using this element in an upcoming effort and will value having the full gradient.

I'm also enforcing the initial colour as var(--white) regardless of theme, for better contrast and visibility.

Before:

After:

Test plan

This is used anywhere the default user avatar is displayed.