Changing theme not working

I've installed a fresh copy of 1.5.6 and duplicated the 'default' theme, renamed it to 'custom', made a few small changes to the view\theme\custom\stylesheet\stylesheet.css and uploaded the file. I also made some changes to view\theme\custom\template\common\header.tpl

I only have one store setup in admin. I've then gone into the admin panel and changed the setting for System > Settings > Store > Template to 'custom' and then clicked save.

However, when I view the site it's still loading the 'default' theme. Am I missing something basic here?

I forgot to mention, if I view the mysql value for oc_setting.key = config_template, the value is custom which tells me the DB has the correct theme stored but for some reason it's not being applied.

I managed to fix it by duplicating the default theme again and uploading it under a new name. Why it didn't like the first theme name I don't know!

I'm also experiencing a similar problem :
when I want to change the theme on localhost nothing happens. Then I uploaded all my files to my online server and there the theme changing works well.

On localhost I'm on a new installation of Ubuntu 12.04, with no special parameters on apache (see my phpinfo). Also I tried the theme changing on a new opencart 1.5.6 installation with no vqmod or any custom stuff.
That's very strange, I have no idea of what is the cause of this problem.