I'm really not sure how else to explain this.
The application background does of course change with the Office theme.
BUT that isn't what the OP asked for! He/she wants to make the report background less bright in DESIGN view ONLY
Perhaps some screenshots will help. This is the same report in design view with the Black, White and Colorful themes.
The theme affects the interface but the background of each section of the report is unaffected as that is set within the report properties
The OP can of course alter the background colour of each section but that will also affect the appearance when printed.
Access has never had separate settings for print and design, This is the first time in 25 years I've ever heard anyone indirectly ask for such a thing.
Meanwhile, the solution I gave in post #3 does what the OP asked for.
Unfortunately, we're now at post #12 and the OP appears to be AWOL