If you'd prefer to make minor changes to an existing theme rather than creating a new one, follow these steps. By doing so you'll ensure that when you upgrade your Gallery, your theme edits won't be overwritten. Gallery's theme system will load style sheet and template files from folders named 'local' instead of the theme's core files. Just make a 'local' folder then make a copy of the file you wish to edit. The following steps are specific to the matrix theme—just replace matrix with your chosen theme (siriux, floatrix, carbon, etc.).
Important: If you made some changes, but you still can't see the change:
Be sure to flush caches (under siteadmin/maintenance) and/or turn off page level caching (under siteadmin/performance) before you try debugging deeper.