As mentioned in Firefox themes in an overview, the ability for users to theme and restyle Firefox is one of it’s greatest assets, together with addons. But here are a few usability notions to keep in mind when designing themes.
- Try to reuse conventions. Doing so increases user productivity and self-confidence.
- Buttons need to stand visually apart from each other and the rest of the window so that users have it easy remembering which is which. Use different colors and shapes to set them apart.
- Buttons need to be large. Being located on the periphery of the browser window, users need to move their pointer some distance before reaching them. Remember that pointer precision decreases greatly with greater distances.
- Favicons help remember the position and distinguish between links on the bookmark toolbar and in the menus. Themes should not hide favicons because they greatly speed up productivity.
- Apple Human Interface Guidelines
- GUI on Wikipedia
- Eclipse (Development platform) User Interface Guidelines
For more information, search for the terms « Usability », « User Interface », « Human interface », etc. combined with « Guidelines » or « Recommendations ».