Function getCssVariable

getCssVariable(
    name: string,
    fallbackColour?: string,
    root?: HTMLElement,
): string
Gets a CSS variable.
```
// Fetch --accent variable, or use `yellow` if not found.
getCssVariable(`accent`, `yellow`);
```
Parameters
- name: string
  Name of variable. Leading '--' is unnecessary
- fallbackColour: string = ...
  Fallback colour if not found
- Optionalroot: HTMLElement
  Element to search variable from
Returns string
Colour or fallback.