If you changed themes with the Theme Editor, it's stored in the server. Restarting it don't remove your changes. If you changed the corresponding source files, you have to build them, copy the .jar in the server and restart it.

Also, you can use Nuxeo Studio if you need branding customization.

The way themes and styles are working in Nuxeo DM are a bit complicated for now, that's why we recommend to use Nuxeo Studio. As it will be simplified, we don't want to add too many details in documentation.