Make a custom CP theme? [EE4]

in EE2 it was possible to create a custom theme, and inside that overwrite files used for the control panel e.g. creating a file:

themes/cp-themes/*custom*/_shared/header.php

…and that would overwrite the file at:

system/exp..ne/views/_shared/header.php

but use the other files. I’ve tried similar ideas with EE4s files inside the user folder but not having much luck, is it possible?

I’m not fluent in php, just know enough to copy and paste little bits to get it to work,

so a plugin to do this isn’t a current possibility for me. (know any good tutorials on making plugins for ee out there?)

…is there another way to achieve this?

Thanks