Map Theme Settings to CSS Custom Properties

[johnsfuller]

Since IE is being dropped, I suggest phasing out the theme-overrides.css stylesheet by:

1. mapping all hubl field styles to css custom properties.
2. replace current style hubl values with those custom properties
3. move each section in theme-overrides.css (typography, buttons, form, etc) to its theme partial

See example:

:root {
  --color-body: {{ theme.typography.body_text.color }};
}

body {
  color: var(--color-body);
}

All styles in theme-overrides could be moved back to their relevant partial. This would allow for one main.css file – instead of a main and theme-overrides stylesheet.

Checklist

• I have read the CONTRIBUTING document.
• I have checked the issue tracker to make sure there isn’t already an open issue.