We currently define our 'decorational' iconography in our CSS via
background images and or psuedo content. For coloring these we use
`mask-image` and a background color.
This commit adds a background-color: currentColor to our %with-mask
placeholder that makes the color of these icons default to the `color`
of the current element, meaning the icons now inherit from things like
`:hover`.
This can easily be overwritten as before by just setting the
background-color on the icon manually as before.
dd6506e994