Mastodon usually has a dark theme by default, but you can switch to a light theme or high contrast theme if you prefer.
Go to Preferences > Appearance > Site Theme, choose the one you want and click "Save Changes".
Some instances have additional themes too. Ask your admin if you want more themes added.
@feditips @wisemonkey We do have quite a few extra added 💪
For both Mastodon vanilla and Glitch-soc, you can find them here: https://git.stuxhost.com/mastodon/glitch/-/tree/main/app/javascript
@wisemonkey
Hmm, I think @stux might know?