Setting background color option as if palette names

[dzbyr]

Is there easy/hard way to set just background color bg4 or another palette colors?

let g:gruvbox_background='bg4' "should there a function like this
let s:gb.dark4       = ['#7c6f64', 243]     " 124-111-100

These are not works...

Sorry, couldn't any hint in here.
There are cursor, column, sign column, color column, vertical split, but there is no just background option in there. What am I skipping?