You can change the appearance of the InsaneJournal website by choosing one of several site schemes. A site scheme is basically a "style" designed for InsaneJournal system pages (i.e., a page that is not a journal or friends page). There are several different site schemes that are available for use. These site schemes were developed by users and made available for public use.
To change which site scheme you use, click the "Site Options" link from the "Edit your settings" heading on any InsaneJournal system page. (Currently, some of the site schemes do not have this link under the "Edit your settings" heading. If this is true for the site scheme you are currently using, use the following URL to change your scheme: http://www.insanejournal.com/manage/siteopts.bml.)
Once you are at the Browse Preferences screen, select which site scheme you would like to use and click "Switch Scheme". The page should automatically refresh with the new scheme, making it possible to select a different one, if you wish.