Cannot get background of greys on Mac Sierra
Hopefully one of us can help...
First, can you clarify what you mean by "Backround of greys"? Do you mean you want all the panels, property bars, toolbars, etc. to be a grey? Do you want it to be light, medium, or dark grey?
These settings are found under Preferences> Interface > Color Theme. ***Note that you must restart Painter for the UI to change colors.***
Or do you mean the color _behind_ your document? This is under Preferences > Interface > Workspace Options > Background Color. Note that this will only change the color behind your document, and you will only see this when the document is 'zoomed out'. A restart is NOT required for this change.
There are possibly other reasons why you are not able to get the colors you want, but we would need more information...for example a screenshot would be very useful...