FediTips , (edited ) 
You can use Mastodon with dark mode, light mode or other themes.
On mobile apps this will be in settings, usually under "Appearance" or "Theme" or "Colours".
If you're on your server's website or web app, you can do it this way:
- Click ⚙️ Preferences
- Click the dropdown menu labelled "Site Theme" and select the theme you want
- Click the "Save Changes" button
Some apps and server websites will also have even more options.
More questions answered:
➡️ https://fedi.tips/customising-your-mastodon-interface-colours