Hey Amin,
Regretfully, changing color for the header only is not available in X. The stacks in X are design sets and the level of customizations or options offered are anchored to the stack’s design. So for Integrity Stack for example, it offers light and dark design. You can’t make the header dark while the rest of the parts are white.
For that, you’d need to upgrade to our Pro theme. One of the reasons why Pro was built is to give more flexibility to our users who want a custom header and footer. It comes with a Header and Footer builder, by the way.
If you don’t want to upgrade to Pro, your option would be to learn CSS and also learn how to use the browser’s element inspector. This way, you can customize the appearance of X where there’s no option offered.
Hope that helps and thank you for understanding.