IMPORTANT: Please note that any changes done to the Theme may affect all the sites that are using it. We suggest to Clone the Theme first, then use the clone version to make the changes so as not to affect other sites.
If you have the right permissions for your site, you can easily change the background color to whatever you like.
From the editor panel on the left pane, choose Theme Editor
Select the theme that you want to change and click Edit
Search for Body Background Color using the Search Bar at the top left side.
Input the Hex code for the color that you want
Click on Publish
Check your site for the updated background color
NOTE: For checking a color's hex code, you can easily search google for it or you can use sites such as Color Hex.