I'm just interested to see if there's any way of doing this, specifically for discord. Yes, I know discord doesn't care if you use a modified version - I'm just curious if you could use a theme without breaking their tos at all
No. If an app doesn't support theming itself, there almost definitely won't be a custom app providing theming option for it. As a web app it definitely has theming options though, at least the browser version. There are various extensions that let you modify CSS of websites, like Stylus or Stylish. The desktop one might be packaged in a way that doesn't let you modify CSS
But your best bet is, as you mentioned, using something like BetterDiscord that supports theming.