theme-ui design system site

[atanasster]

Added a few more features to help test theme-ui components

Here is an example of the Message component rendering on various themes, with seemingly some issues with the dark and deep themes color contrast:
https://theme-ui-design-system.netlify.app/docs/components-theme-ui-message--overview/themes

Also started adding the design tokens, with colors coming first:
https://theme-ui-design-system.netlify.app/docs/design-tokens-colors

Would love some feedback and what else would be good to have (currently working on fonts, spacing design tokens), repo is here https://github.com/atanasster/theme-ui-design-system