That should be placed in your theme's qa-theme.php file. You can find it in /qa-theme/<your-selected-theme-folder>/qa-theme.php. How to "change backgrounds" I guess you mean CSS changes, which you should do from a qa-styles.css file in your theme's directory. Anyway, that is actually something that theme developers choose how to do, so maybe that file doesn't even exist. All themes in the Q2A distribution come with that file, though.
