Okay, I'll add the quit option to my list.

You can replace all the icons and graphics in BetterZip, like in most other Mac programs. They are just png and icns files inside the application bundle. To view them right-click the BetterZip icon in Finder and select Show Package Contents. Then navigate to the subfolder Contents/Resources. You can edit or replace all the files, if you like. (The graphics are of course not public domain, so you can't redistribute them.) Just be sure to store a backup of your files in a separate place, so you can re-add them after upgrading to newer versions of BetterZip in the future.