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.
@trinsec
Credit to https://www.reddit.com/r/ProgrammerHumor/comments/mpa2gd/the_heck_codes_in_light_mode/
@feditips