Polarion is run as a service on the polarion server. The service really respondig to your browsers request is the bundled apache web server. It has three different locations configured with different methods of answering:http://yourserver.yourdomain
gives you the content of the htdocs folder - in most cases and even here apache shows you the index.html file.http://yourserver.yourdomain/polarion
is forwarded by apache to the polarion service that shifts bits and bytes and serves you the graphical interface.http://yourserver.yourdomain/repo
finally gives you access to the files in the SVN repository.
To customize the interface - you will have to keep track of everything you change as it is lost with the next update - you can navigate to %POLARION_HOME%/polarion/plugins/com.polarion.alm.ui_<version>/war/gwt/polarion
Ther is a bunch of css files that are used on every page. Ignore the buildId and add your browser to the filename. Then you get the right css file.
When you want to change styles in your own widgets or on some pages only, you can include style definitions in the wiki-code.
If you want to change the original css files, you should try to find a way to import your own file (in htdocs for example) to only maintain the include.
Or you get your users to like the (I think it's okay) original design of Polarion. Don't waste your time on design and start using the great features.