In Site Manager > Sites > Edit Site you can assign an editor stylesheet to your website. In this stylesheet, you should only put the styles for your content such as your H1-H6 tags, paragraphs, links, and so forth. Also in this stylesheet, make sure you set the body background color to the color you wish the html editor to have.

Hello,

Just for your information, this is also answered in our FAQ section. Please check our Knowledge base and FAQ section at first before asking questions. There are already a few topics covered in there and you'll get your answer faster.

Best regards,
Boris Pocatko