How to allow your vBulletin members to change the forum style?
To allow your vBulletin members to switch between the installed forum front-end styles, navigate to your vBulletin admin panel > Settings > Options > Style & Language Settings > Edit Settings.
Select Yes in the Allow Users To Change Styles area and save the modification. In this way when you log in your forum’s front-end you will see the drop-down menu in the bottom left corner and you will be able to change the styles through it.