Very easy, there are only 2 steps!
Add the file
First, you can add your .css file into your custom css/ directory.
In the default nuxeo DM sources, it's here:
For your project you probably have a custom nuxeo-platform-webapp (for every pictures, icons, custom xhtml...).
Add your new css/ directory with your custom .css file there.
Declare the file
Then, declare your css into your custom theme-contrib.xml file:
Copy and paste the following theme-contrib.xml file to the OSGI-INF/ folder of your project, register this file in your META-INF/MANIFEST.MF and customize it to point to your actual .css resource:
Note the merge="true" option that tells the theme engine to merge this view contribution with the existing nuxeo5 includes as defined in the default Nuxeo theme. Don't forget the <format-type>widget</format-type> statement that is required to apply the override to the right view (the format-type is used internally to identify the view along with the view name and template-engine attributes).
Relaunch you app, restart your server and it's loaded.
If you prefer declare your own fragment, you can also write:
Then you have to place your custom fragment into your custom theme file, in every page you want to have it (2 pages by default, "Default" and "Dashboard").
Relauch your app and restart your server to be sure all your changings will be loaded.