Aria-expanded="true" how to use in LESS?

Question

HTML

Good afternoon to all makers!
I'm laying out an accordion using LESS and I want to change the attribute property (Aria-expanded from "false" to "true") in the button's styles to trigger a certain rule.
In vanilla it would look like this:

li{
&.tree-parent{
  &:[aria-expanded="true"] {
     background: url('some-image.gif');
     }
  }
}

but less does not compile this way.
Hope for your knowledge!
Thank you for your attention.

Answer 1

li.tree-parent[aria-expanded=true]
Try this.