Clementine - Post Background Color

The post background colors will be the color of the Content Background color. You can edit this color in:  Appearance > Customize > Posts/Pages Layout > Page Layout > click the "Design tab" at the top > edit "Content Background" color at the very bottom

*Please note that in the page editor, the background color of the post content will be white (like the image below) but on the live site, the posts will have a background color. 

If you want to change the background color to white on any post block (while keeping the normal color on other posts in the settings), you can add this custom CSS class to Additional CSS in the post block settings:

Use:  white-post-background