How can i change the colour scheme for my blog post background and header independently?

Hey guys, 

Im looking for some code to set the background colour scheme for individual blog posts without affecting the header. 

I have a transparent header that turns dark when scrolling down (see pics), but i want the background of the post to be white.

by the way im using 7.1

here is the post https://bellflower-tuba-xfw8.squarespace.com/blog/dummy

or maybe better, could i just invert the logo colour in the header only when scrolled down, and only for blog posts?

Hi,

Yes this is possible. But your site is private. Can you setup an access password?