Rework imported css variables from spectrum-css

[benjamind]

Expected Behaviour

Since our CSS is 'modern' and has all the css variables from spectrum intact and not processed out we should be able to include a minimum set of variables for medium/large/light/dark themes.

Actual Behaviour

Currently we include all the CSS produced by spectrum in our themes and this results in a huge amount of wasted bytes.

Reproduce Scenario (including but not limited to)

Load up the documentation site and inspect the imported css.

[benjamind]

@cuberoot I took a quick look at this and I think it should be relatively easy to modify the css processing to import the theme files (medium/large, dark/light and global/core) and wrap them in :host, :root selectors so they can be imported into root elements which will then apply styling down the tree. This should massively reduce the CSS payload.

[msdewey]

closing issue, resolved by #87