DevTools/Developer Edition Theme
The Developer Edition Theme is implemented as a lightweight theme, but it's different from others in a few ways.
1. Rather than being installed separately, it is bundled with the browser using LightweightThemeManager.addBuiltInTheme. Because of this, it's only available (for now) in the Nightly and Developer Edition channels (see Bug 1181721 for more information).
2. It has an extra stylesheet that's loaded when it's applied - see the CSS Section for more information. This gives it more control than other lightweight themes, which allows it to override a number of CSS variables and other styles.
3. It changes colors when the DevTools theme changes. By default in Developer Edition, the DevTools theme is dark. In Nightly (or local builds) the DevTools theme defaults to light. This is controlled by the devtools.theme pref.
How To Apply
It's easy to test changes to the Developer Edition theme. You can apply the theme by selecting it from Addons Manager -> Appearance, or from Customize mode.
CSS
The main styling for the Developer Edition theme is located here: http://dxr.mozilla.org/mozilla-central/source/browser/themes/shared/devedition.inc.css
Then each OS has their own files for platform-specific styling, located here: