Here are steps to apply custom style to OBIEE dashboard.
1. Stop OC4J
2. Go to folder Drive:\OracleBI\oc4j_bi\j2ee\home\applications\analytics\analytics\res and make copy of folder s_oracle10 with some different name. For instance, s_mycompany.
3. Modify CSS and images in the subfolder b_mozilla_4 under s_mycompany, as per the requirement.
4. Copy s_mycompany folder to Drive:\OracleBI\web\app\res
5. Start OC4j and Log on to analytics web application.
6. Go to dashboard properties and select newly created style "mycompany" in the style dropdown.
The screen below shows default OBIEE banner area.

Here I made changes to banner images and styles to change the looks.

You can write to me if you want files for above style.