In the very first Symbian Series 60 v2.0 phone, the Nokia 6600, it was possible to select iconsets independently from other theme elements. You could select the icons from one theme, and the other theme elements (such as the background and battery meter) from another.
Unfortunately, this functionality was removed from subsequent Nokia handsets; meaning that when you select a theme you are stuck with it's icons. Symbian Freak has a guide on how to re-enable this functionality; basically you have to replace the .ini file of the theme you want to use with the ini file of the theme's whose icons you want to use. (I think.)
An alternative would be to edit the ini file directly. Good luck if you're trying the change! (I'm too lazy, myself.)
Note: All currently shipping Nokia Series 60 handphones use v2.0; including the 3230, 6260, 6620, 7610, 6680, 6681/6682.
