I may be showing my ignorance here, but yes --button-card-light-color is for lights, I was just using a variable with a predictable colour. I’ve since changed it to ‘goldenrod’ for testing purposes.
You suggest entity.attributes.percentage rather than variables.circle_input? if so, results are the same - the only time the background color updates is when I close/reopen the popup.
I’ve attached a capture of what I am experiencing (sorry for the quality, first time doing this)
edit: I think it has something to do with the browser_mod popup not refreshing. I took all my buttons outside of the popup and the buttons behave as I expected them to.