Please see the reference guide.
I have done some simply work.I can share it.I create WAR file with these contents:
1. a directory called images with some pictures in it.
2. a directory called WEB-INF with two files,portal- themes.xml and web.xml in it.
3. a file called my_style.css.
The content of portal-themes.xml is required to make some configuration,example:
The content of web.xml is empty.
I place the war package in the root directory of portal.When portal is deployed,you can see the effect.