Editing Headers for themes 2024 0r 2023

Is there a secret to editing Headers in these themes in particular? Or in any theme?

I find the headers in the block themes and try to do sth basic like changing bg-color. In 2024 theme, nothing changes. I can change other elements, add shadows to them etc., just not the header. I thought, maybe it's a bug specific to me in my theme. I switched to 2023. Now it's even worse; I attempt to change the header, and everything else changes in color except the Header. Like the style is being passed down to everything below the header, just not the header itself. This is highly confusing.

I am just trying to achieve a high level approximation to what I have in mind, tinkering basically. This is the first thing I try and nothing works.

Halp appreciated