Redmine

Allows to set a visual style per project using project identifier. Useful when hosting projects from different organizations with different visual identities.

Checks if there is a theme with the same name as the project id and loads its application.css in the header.

You can see it working here:
https://comunidad.alial.es/projects/alial (yeah, that's redmine extremely themed :) ) If you go to https://comunidad.alial.es/projects the header is different.

The plugin is so simple I didn't put it in any code repository.

As any other redmine plugin, place the archive contents in vendor/plugins.

Notice you must have a theme directory and an application.css file inside. The directory must be named as the project id.