IE11: Add fallback to CSS vars

# Bug report

- [x] This is an issue!
- [ ] Is that a question!

## Summary
The color modes of the vue-a11y website use CSS variables and IE11 does not support it.

## What is expected?
Choose themes in IE 11

## What is actually happening?
Not displaying the correct colors

## Additional Information
- https://vgpena.github.io/winning-with-css-variables/#easier-fallbacks-with-sass