2.**Add your themes to the config.** This is what [the default themes.yml](https://github.com/tootsuite/mastodon/blob/master/config/themes.yml) looks like in Mastodon. To make your custom themes visible in settings, you need to add a new line in the form `themeName: path/to/theme.scss`. For example, the modern-dark theme would require adding `modern-dark: styles/modern-dark.scss` as a new line.
3.**Add a human-friendly name for the themes (optional).** You can edit each desired language's locale file in `config/locales/[lang].yml` to add a localized string name for your theme's `themeName` as added in the previous step. For example, [the default `config/locales/en.yml`](https://github.com/tootsuite/mastodon/blob/041ff5fa9a45f7b8d1048a05a35611622b6f5fdb/config/locales/en.yml#L942-L945) contains localizations for the three default themes that ship with Mastodon, into the `en`glish language. You need to do this for every language you expect your users to use, or else they will see the unlocalized `themeName` directly.
4.**Compile theme assets and restart.** Run `RAILS_ENV=production bundle exec rails assets:precompile` and restart your Mastodon instance for the changes to take effect.
