Change background colour of specific H2 titles

You are talking about the headings ALB elements – right?
you had to know that this custom class ( ID’s have to be unique – so if you want to use it on a page twice – a class will be better ) goes to the parent container so selector will be before the h2 container itself.
and if you do not like to have the whole width with a background-color – you had to set the container to display : inline-block
( my test custom class was: heading-bg-special)

.heading-bg-special .av-special-heading-tag {
  background-color: red;
  display: inline-block;
  padding: 2px 8px !important;
}