Please refer to technical documentation pack of yur version. There you'll find a section how to customize web app theme. Maybe this helps you.
The file which contains the web theme should appear something like this:
You can decompile the jar online, with the dev kit, or switch the '.jar' to '.zip' to begin editing files. The changes you're interested in sound simple enough - just browse the jar structure for the appearance settings file. Specifically I believe it's this file:
Just make sure to review any differences when installing PIM hotfixes or upgrades!
Need to dive into this, but this seems to be the place indeed.