For certain icons, it can be near impossible to tell which window is selected (example above). This also happens for the preset icons - because they're thin, it can be easy to miss the faint accent color tint.
I am suggesting that the background of the icon should be highlighted, which makes it extremely obvious that the window is selected.
I expect this to make it faster for users to tell which icon is selected - instead of scanning to see which icon is accent-color or which emoji is non-grayscale (which is a bad indicator for emojis that are naturally grayscale), the user simply has their eye drawn to the icon with a background.