To backup a style to work on a test template, go to Billing Manager > Tools, and go to the section where you want to work.  Find the template on which you would like to work, click it and then press Edit.

Now that your style is opened, go to File > Save to File.

Name the file and save to a location you would remember.  The file will have a .RTM extension.  When done, close the report template that is open.  Now, back on the Invoice Styler selection screen, choose New.

This will open up a blank template.  Go File > Load from File.  Point it to the file that you just saved and press Open.  Now go to File > Save as and name it Test.  You can now use this template to test any changes you want to make.