Andy,
You can set it as your default by going to the User Control Panel (User CP) at the top of the page
User Control Panel
Select "Edit Options" on the left side of the screen.
Scroll to the bottom of the page, and it will say, "Forum Skin" just above the "Save Changes" and "Reset Fields" buttons...make your selection and then click "Save Changes" and you should be good to go...
Let me know if you can't get it
