OBS Studio has the ability to load different themes based on your needs. The option is available in Settings -> General.
Default themes includes:
The files for the default themes can be found in the OBS Studio installation directory, specifically /data/obs-studio/themes/
The best place to install custom themes is in your user directory - the same place where profiles and scenes are stored, so that if you ever reinstall OBS Studio, you don't lose your custom themes.
You may have to create the themes directory yourself.
You can find themes created by members of the community at this link.
Themes use a combination of something called QSS and a folder of custom images.
QSS is based on a subset of CSS2, and its documentation is all over the place.
It's recommended to open one of the default themes (Rachni has great documentation) and use it as a reference.
Documentation for QSS is split up quite heavily, but here are the links you will need: