Select the Design tab on the ribbon
Find the Page Background groupand then Page Colour to see the options
Select the colour you prefer to work on. You can hover over the colours to test them, or simply keep changing the background until you find your preference
Note: if you print, the print out will not show the background unless you go to File > Options > Display, and tick Print background colors and images > ok before you send your document to print
The colour on the screen may not exactly match the printout. If you prefer to read on an exact colour background, it may be better to print on paper that colour rather than trying to print the colour.