In ACP -> Style Manager
Do you see the styles you have installed and do they have the check box checked.
Checked = Usable
Unchecked = Usable by Admin only
Check your forum permissions in
Forums & Moderators -> Forum Manager
Select the forum/s -> Edit -> Style Options
- Custom Style for this Forum = Use Default
- Override Users' Style Choice = no
and what about the suggestion from Opserty ?