CSS Variables and Consistent Sizes

[Reisen]

Using create-react-app gives us CSS modules and the option of using a CSS pre-processor. While I don't want to bring in any pre-processors I do want to bring in CSS variables for easy consistent theming and sizing. The goal of this ticket is to see how hard it would be to be able to get some kind of web/src/theme.css that would allow defining:

@background: #EEF;
@link: #3366FF;
...

To use in other CSS files.