Yes, in the Modeler you can select a theme package in settings -> project settings -> Theme package location.
You can create theme packages yourself. We have a manual available explaining how to do this. You can download the manual here (pdf).
Yes, you can.
The UI for any given web client is based on a simple HTML index file with CSS import statements and a few layout elements. You can change layout in this file and override any CSS by importing your own CSS file(s).
If you package your own files in a ZIP file (generally referred to as a theme package), you can include it in the deployment phase by following the steps Johan mentioned.
Thank you for the awnsers but i still don't know where I can find the manual.