To set a default style, visible to guests and users that are not logged in, perform these steps;
1. visit the ACP
2. on the left menu find the section 'General Admin'
3. within this section select 'Configuration'
4. on the right pane scroll down, if required, until you see 'Default Style' in the left column
5. from the dropdown list in the right column select a style you wish to use as the forum default
6. Optional: If you want every user to use the style set in 5 then set the option 'Override user style' to 'Yes' this will then use the style set in 5 for all users regardless of the style they have selected in their profiles
7. Scoll to the bottom of the page and click 'submit' to update the forum configuration