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, change the related variables in the theme import file. You can set all to 999px for circle avatars.
SkeuoCord now supports setting an image as the app background. Edit the theme import file to add the link to the image you want to use. (You will have to download the theme import again to use this feature if you downloaded the theme before this update.)
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.