Themes and js modification(client side only)

over time you guys have added some really nice features to the apps and website, but when it comes down to themes and cool javascript effects, we have to use things like betterdiscord which violate TOS, why can't there be a built in editor, or a whole other program to modify discord to our liking?, it's just an idea but i think it would be a really unique and cool feature that you don't really see on other apps or sites.