Function getCssVariable

Gets a CSS variable.

// Fetch --accent variable, or use `yellow` if not found.
getCssVariable(`accent`, `yellow`);

getCssVariable(name, fallbackColour?, root?): string
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.