Customiser: Color preview with color variables

[lungyiing]

Using javascript to automatically update a small preview for colour variables on the customiser (perhaps using input groups).

For example:

[cvrebert]

Duplicate of #11904.

[cvrebert]

I'm thinking that we could use an <input type="color"> polyfill for this.
My current polyfill recommendation would be https://github.com/bgrins/spectrum

[mdo]

Hah, having just played with Spectrum for something else, fuck that. Maybe we do just rock the color input and make that an enhancement for folks in particular browsers?

[mdo]

Only latest Chrome and Firefox support the color input FYI.

[cvrebert]

We could just leave it as a text field and add a preview swatch (like in the above mockup). Would be pretty easy. I'd guess most folks are gonna copy-paste in colors anyway, rather than coming up with them on the spot.
Slight caveat with any of the approaches is that they don't let you preview Less color expressions (short of adding a good chunk of JS to hook into the Less compiler more).

[mdo]

I think given that this makes the most sense to save for v4 or v5 when we can simplify things a bit more.

[mdo]

Closing and punting to our v4 list.