Clean up rheostat css theming (3/7)

[philipfweiss]

This commit sits on top of #162, and will be rebased when that commit is merged.

Cleans up CSS/ the default theme for Rheostat.

[ljharb]

with theming, would we be able to define these values in the theme, so users could define their own default handle width?

Separately, where does 1.9 and 0.8 come from?

[majapw]

I think we might want to keep this as color.focus and just have it map to teal in the theme so that the variable is meaningful as to what it represents.

[majapw]

nit: lowercase

[majapw]

LGTM after we squash down these two commits (they don't seem very meaningful separately)

[majapw]

We do need to remove clean-css from the deps, but my other comments are probably not entirely necessary. Just worth thinking about.

[majapw]

this looks like a mistake

[majapw]

I think here this should maybe be color.progressBar and point back to the teal? I don't know that there's an inherent relationship between focus state and progress bar color and we should the ability to override individually in the theme.

[majapw]

I think what I meant instead is that we would have:

 teal: '#abc4e8',

and then also have:

focus: this.teal,
progressBar: this.teal,

or whatever.

Alternatively, you could have:

const core = {
  black: 'black',
  white: '#fcfcfc',
  grey: '#d8d8d8',
  teal: '#abc4e8',
  lightgrey: '#dbdbdb',
};

const DefaultTheme = {
  ...
  color: {
    ...core,
    focus: core.teal,
    textDisabled: core.lightgray,
    ...
  },
};

but maybe that's overengineering it and not super helpful.