background: inherit!important;
background-color: rgba(255, 255, 255, .3)!important;
backdrop-filter: blur(5px)!important;
I would try adding these lines in below where you've made it a pill shape, I assume within
header {
}
The 255, 255, 255 values can be changed to a custom RGB value, and the .3 can alter the opacity of the header. The blur number in the next line can also be changed.
Cheers
Fin.