Using the Theme Editor under Mozilla FireFox 
Note that the following functions that are available under Microsoft Internet Explorer do not work under Mozilla FireFox:
The Undo/Redo and Reset Parameters functions are not available.
The Options link next to the input fields for the parameter values, is not available.
The preview of the modified style parameters is not performed automatically. You therefore have to update the preview each time.
Note
You can find the most recent information about restrictions under Mozilla FireFox in SAP Note 960549.
Recommendation
We recommend that you modify the theme in Microsoft Internet Explorer where possible. Use Mozilla FireFox mainly to check, and if necessary adjust, any theme changes you made.