CSS Customization Guide - itch.io
Confirm that you will not alter itch.io’s built in UI with your CSS and that you will scope all changes only to the visuals of your page content.
you alter the header bar default color...
195 days ago (+2)
After further investigation with itch, the issue is with Itch itself.
As you can see, Edit Theme -> BG controls the Header Bar colour.
Many thanks for raising this concern!
It seems someone internally has accidentally added a new background colour styling, altering core itch.io code! 