Replacing `filter()` with css variables causes regression

[alex-page]

These filter-color values passed to the 3rd argument of recolor-icon are sent as the filter property. They're not actual color values, they're filters like brightness(0) saturate(100%) invert(100%).

I think changing these to colors is going to break the behaviour, which is to allow for the recoloring of icons that are passed as svg datauris and rendered with an img tag

Originally posted by @BPScott in #4650 (comment)