Added a basic theme picker to the Masto-FE (🦥 flavour) thingy, it's under app settings. Just light/dark/dark (high contrast) available rn, as that's what was already in the glitch repo.
As always, you can try it out here: https://masto-fe.superseriousbusiness.org
Code here (and I added a brief readme now): https://codeberg.org/superseriousbusiness/masto-fe-standalone