Comment 6 for bug 549552

The new radio button would be for providing a way to switch back to the system/theme default transparency. However, such a button is not provided by most other dialogs/apps either, so I see that rather as a new and independent feature, than a necessary preliminary to fix this bug. The theme could just provide the default slider position and radio button ('transparent bg'), and the user can customize it as usual.

If I got that wrong, can you please give more details about "make it behave the way that users expect"? Thanks!