* ui: Remove custom css-vars 'polyfill' and use native CSS props
Previously we used a sort of polyfill for certain places where we needed
CSS property-like behavior. This meant duplicating code between JS and
CSS, specifically some of our SVG icons.
We moved to CSS props only in the places where they are beneficial and
populated the variables with our already existing SASS variables.
This means we no longer have to duplicate CSS and we can remove our
custom css-var helper/polyfill.
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