You can give your Mastodon server’s website interface a unique appearance by using the Custom CSS feature:
- Log into your server’s website using the admin account
- Go to Preferences > Administration > Server Settings
- Click the Appearance tab and go to the Custom CSS section
- Add the CSS code you want in the box, then click Save Changes
If you’re comfortable editing CSS, have a look at this excellent tutorial for customsing a Mastodon server’s colour scheme ⧉.