After:
This add-on lets you customize avatar markup up to your liking (i.e. how avatars should look like). By default it only adds a complimentary gradient, which can be changed in Style Properties → Avatars → Dynamic avatar gradient color function.
If you want to further expand on customizations feel free to edit avatar public template.
Please bear in mind that it doesn't use any caching yet, so it will slow down your forum performance.