Color properties can fail using keywords

[dkrasner]

Main issue

Property values are set irregardless of whether the property is at all valid. In general this is fine since we should ignore invalid property values but with our current setup for colors, and how css is updated that will actually break the app.

For example set the color on some property to a keyword which is not referenced in the ST color dict. You'll notice that nothing happens visually but if you look up the prop it will be set like so:

temp0.model.partProperties.getPropertyNamed(temp0.model, "border-bottom-color")
"grey"

When we try to update the transparency prop or reload the snapshot, or do anything that depends on a RGBA values the app crashes.