Hi.
Im using 3.1 and new to moodle.
I got this problem following the "Creating a theme settings page" tutorial(which uses 2.x), and that was because my own newly created custom themes config.php had 'standard' theme as one of its parent themes, but the 'standard' theme doesn't exist in 3.1, it didn't give a error message or anything, so took a little while to figure out.