I am not sure about such a feature....
But it should be do-able:
components\synedit\synhighlighterpas.pp
in
function TSynPasSyn.GetTokenAttribute: TSynHighlighterAttributes;
check
if rsInterface in frange then ...
then modify the background...
To make that work you need an extra object (just like FCurIDEDirectiveAttri).
Assign it, alter it, return it.
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That is hardcoded. Adding it to the IDE options is a bit more work