Another option to render all user-selected stuff irrelevant is changing coppermine's cookie name in coppermine's config. This will do that same as if you deleted all cookies from your users. They all will have to log in once more.
But imo the original question deals with something else: Andy seems to have issues setting his custom theme as default theme. The reason is that he played with his own cookies, making another theme the default for him only. Real simple solution is to access coppermine's config with the new custom theme as URL parameter. Let's say your custom theme's name is "bill", then just access
http://gallery.svekaschoten.be/admin.php?theme=billAnother thing that might cause problems is capitalization or illegal chars in a theme name (i.e. the folder name of your custom theme). Make sure that it is all lower case and only contains alphanumerals. Only allowed special chars are the dash and the underscore.
@Andy: post the name of your theme. Ah, strike that. I just went to your themes folder and realized that you haven't set the proper index option in .htaccess, so one can access your folder structure just fine (which is not a good thing usually, but it helps when supporting). So your custom theme is named "svekaschoten". Do as I suggested and access your config page with this theme name:
http://gallery.svekaschoten.be/admin.php?theme=svekaschotenYou should be able to pick that as default. If not, rename your cookie, then try again. Make sure to use a valid cookie name as well (in fact, the same naming scheme I suggested above for theme folders applies to cookies as well).