Some of our themes such as Japan allow for you to choose a solid color as a background for your posts.
To start click the 'Edit theme' icon on the top right of your logged in home page to open the edit side panel.
Look for the following panels:
'Posts background color' - this is where you can choose a color
'Posts content style'- if you've selected a dark color you'll want the post content to be 'light' so you can read it and vice versa.