Hi Sidhaarth
1/If you are looking to set the background color for your visuals . you can customize and build your theme and set up the primary and second color of choice but this will be for all visuals.
2/In case you want to set it up for selected visuals, you can pick free-form layout and set up background color within style property of your selected visual . Here is the link that would help
Also we have one blog supporting similar scenario - Create stunning, pixel perfect dashboards with the new free-form layout mode in Amazon QuickSight | AWS Big Data Blog
Please let us know if this information is helpful.
Thanks
Neeraj