If you’re using Mastodon through the website, you can choose your interface colours by going to Preferences > Site theme, choose the theme you want and click Save changes. By default Mastodon servers come with a dark theme, light theme and high contrast theme, but some servers have a lot more options. If you want more themes, ask your server administrator about it.
If you’re using Mastodon through the official app, you can choose interface colours by clicking the cog gear icon ⚙️ and select Automatic, Always Dark or Always Light, then click Done. There’s also a toggle switch to activate “True Black”, which gives the dark mode a much darker background colour than usual.