Replace Sass variables with CSS custom properties

### Your use case

#### What would you like to do?

Replace all Sass variables with CSS custom properties.

#### Why would you like to do it?

- Improve the performance of theme switching
- Improve CSS compilation times
- See actual variable names when inspecting the page instead of hardcoded values
- Potentially simplify the custom themes implementation?

#### How would you like to achieve it?

Separate our theme files from the rest of the CSS so that they can be swapped out easily.

### Have you considered any alternatives?

Not doing this if we have a good reason to continue using preprocessor variables.

### Additional context

https://caniuse.com/css-variables