Hi equilibriumx
You can use this custom css:
#gkPageContent {
background: #FEFFE8 !important;
}
In case that you want to apply for specific menu item, you can set a suffix class for that page via template >> Features tab >> https://prnt.sc/s26jhs
For example, I add a new class suffix called newbg for a menu item ID: 419, then I will override with this css rule:
.newbg #gkPageContent {
background: #FEFFE8 !important;
}