Hi everyone!
I think i fixed this. It seems to be very simple.
The problem is that when you set the option to be "Do not force" an empty value is set, but in course/editcategory.php, you have this near line 64:
if (!empty($data->theme) && !empty($CFG->allowcategorythemes)) {
$newcategory->theme = $data->theme;
theme_setup();
}
It's where the theme option for the category is set. But when you set it to "Do not force", an empty value is sent to the script, so it will not match the "if" condition "!empty($data->theme)".
I changed this to "isset($data->theme)", which just verify if the variable has a value (empty or not).
So, with just this modification, we have:
if (isset($data->theme) && !empty($CFG->allowcategorythemes)) { $newcategory->theme = $data->theme; theme_setup(); } }
I tested hard after this modification and it works! Now you can revert to "do not force" and have this working 
I attached the modified course/editcategory.php in this issue.
Hope to be useful,
Caio
I can confirm this issue.
Forum discussion: http://moodle.org/mod/forum/discuss.php?d=103715