It looks to be .bases-group-heading if I’m understanding correctly.
so you could try something like
.bases-table .bases-group-heading {
background-color: rgba(242,236,227,0.8);
}
to start. If that doesn’t work, please share what theme you are using and/or the customizations you’ve made so far. Hopefully someone can have a look.
Have a look at the inspector if you haven’t yet.