Please try with Clean and Boost themes. Create or go to a course (with students enrolled). Create an assignment activity (default settings are fine). Go to the Grades page. Go to the Single view report. Select the assignment as the grade item. At the bottom of the page check the "Perform bulk insert" checkbox. Change the Insert value to -1. Click the save button. Check that you see a message for each user that the grade entered is less than the minimum allowed. Check that the last message is "Grades were set for 0 items". Try just overriding one grade with a negative number. Check that you see the same messages about the minimum allowed. Check that the number of grades updated is 0.