Steps to reproduce:

• Login as admin
• Go to Site administration -> Appearance -> Boost
• Upload a dark background image there

• Login as teacher
• Go to a course of yours
• Add an assignment activity
• View the assignment activity
• Click the "Grade" button to open the grading screen

Expected result:

• The grading screen opens full screen
• As the screen opens full screen, I do not see any part of the configured background image
• All navigation items and labels on the screen are properly readable

Actual result:

• The grading screen opens full screen
• Even though the screen opens full screen, some any parts of the configured background image are visible
• Some navigation items and labels on the screen are not readable

Screenshot: