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Building a Theme | MDN

Easiest to create your own theme is to look at an existing theme and then make changes to this theme.
Then you know which icons and CSS rules you need to have.

The built-in default classic theme is stored in the omni.ja! ZIP archive files in the Firefox program folder.

• resource:///chrome/browser/skin/classic/
• resource://gre/chrome/toolkit/skin/classic/