To avoid any confusion over installation of the script, I created this guide:

1. Download SIMBL

2. Unzip the file

3. Launch the SIMBL-..*.pkg installer

4. Download GreaseKit

5. Mount the GreaseKit-..dmg image

6. Copy GreaseKit.bundle to /Library/Application Support/SIMBL/Plugins/

7. Launch Safari

8. Select the GreaseKit item in the menubar
   

9. Select Manage Applications...
   

10. Add Sketchup to the application list
    

11. Quit Safari

12. Download inputPatch.user.js created by driven (his original post)

13. Copy inputPatch.user.js to **~/**Library/Application Support/GreaseKit/

14. Launch SketchUp

15. Select inputPatch from the GreaseKit menu
    

Your text boxes are now white!