Fixes #1109 Themes with same name

Tried a fix for this:
https://github.com/mybb/mybb/issues/1109
No sure if you should use `"LOWER(name)='".$db->escape_string(my_strtolower($mybb->get_input('name')))."'"` here, `My Theme Name` may not be the same as `mY tHeme nAME`.