It seems as though youve imported the new styles and now given your users more options to choose from. upgrading to 7.6.0 does not automatically import any new styles for you. but there are instructions on how to do it manually. this sounds like what had happened.
If you dont want to use any of the styles you previously imported, you can disable them from the style editor, or delete them.
if you want certain forums to use a certain style, go in to each forum editor (in control panel > forums > forum detail > selected stylesheet) and choose the style you want it to use just for that single forum.
note, if you only have one style enabled within the style editor, all the style selection options on the user side will automatically be hidden. ie; "nothing to select from. nothing to show."
