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 I'll never understand why people would ever want to switch to a light theme. That's for crazy people! Just like my admin, he's also a crazy person so I know what I'm talking about! ;)