How to set a ReadOnly background color for the entire editor

While the user is executing a query I make the entire editor Read Only.

I want to change the background color while the editor is in the ReadOnly state.

I see that I can do this by adding a span indicator - but that seems a waste of resources when the span will be the entire document.

I can also do this by looping through all the Highlighting Styles and changing every Backcolor - but again rather long winded.

Is there a way to assign a ReadOnly Backcolor that is automatically applied?

Alternatively is there  a way to overide all Backcolors without actually setting every style separately.
(I thought that was the idea of the DefaultStyle ... but apparently not.)

Thanks
Mike