Now that styles handles community cust, it occured to me that it might be worth altering this FAQ very slightly, so I would propose the following.
Title: How do I customise the look of my journal or community?
New paragraph - between first and second current?: All the options below also apply to customizing your community. In each case, once you are in the appropriate area you should select your community from the Work As box and click switch before making changes. You must be the maintainer of the community to do this. Where a paid account is required, the community account itself must be paid; please see http://www.livejournal.com/support/faqbrowse.bml?faqid=44 for assistance if you wish to purchase paid time for your community.