SkeuoCord gives Discord a skeuomorphic overhaul. Almost every part of its interface, from channels to the Server Boost page, is themed for a complete look. Both dark and light themes (including the Dark Sidebar option for the light theme) are supported.
To customize user avatar border radiuses/remove avatar borders, change the related variables in the theme import file.
SkeuoCord supports setting an image as the app background. Add the link you want to use to the theme import file.
SkeuoCord requires English (US/UK) to be the selected language for button icon theming, due to the way they're specified in Discord. If your client is set to another language, the color of the regular icons will be changed and dropshadow filters will be applied to them instead.
If you don't want to use SkeuoCord's brand color replacement, use SkeuoCord_NoBrandReplace.theme.css from SkeuoCord's GitHub repo.