Currently our disabled elements use cursor: not-allowed; which on hover shows a circle with a slash through it. It's intended to show the click is, well, not allowed, but it's not something folks see all that often I feel. In addition, we don't currently have an easy way to customize this should you want to adjust it.
So, this PR adds the variable and sets it to default instead of not-allowed. This also has the added benefit of fixing a recent inconsistency in behavior—most modern browsers show no cursor at all due to pointer-events. This brings IE10 and below closer in line with that.
Edit: Alright, screw that, we're going with not-allowed still and adding the variable.
/cc @cvrebert @JasperHorn