The front-end interface of LANraragi is customizable out of the box through CSS. A few themes are built-in already. Theme Preference is saved server-wide and will be shown to all users.
Changing Themes can be done alongside all the other app settings. You can write your own themes by modifying the existing ones - Dropping them in the /public/themes folder will make them appear in the selection.
For users who don't have access to the app folder and want to make custom themes, your only option currently is to use a custom CSS browser extension. Docker users can try binding a folder on their machine to the /home/koyomi/lanraragi/public/themes folder.