Polyfill CSSStyleDeclaration methods

[ebryn]

As listed on https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleDeclaration:

• getPropertyValue
• setProperty
• removeProperty

These methods allow the user to programmatically manipulate custom property values at runtime.

I think we'll want to expose these via our own API namespace and offer the ability to augment the actual CSSStyleDeclaration's prototype ala typical browser polyfills.