vB5 does use styles, the system hasn't changed, but there are also some pre-made 'themes' that come with the product.
There seems to be a bug in 5.2.0 regarding styles / themes in the admincp:
http://tracker.vbulletin.com/browse/VBV-15585
There's a possibility a patch might be issued but this isn't confirmed at this point.
The workaround for now is to put the site into debug mode while you need to edit styles.
How to enable or disable Debug Mode in vBulletin 5.x (VB5 Debug Mode)