You can change the interface theme of your Mailfence account by clicking on your username, then choose Theme and select the one of your choice.

Another way to change the theme of your Mailfence account is by following these steps:
1. Settings.
2. Account -> General :

You can choose between Light Theme/Light Mode or Dark Theme/Dark Mode. You can always come back later and change it if you want to.
Check our YouTube video and learn how to change the interface theme of your Mailfence account.