getColor(key) {
    key = key.toUpperCase();

    if (key in this.colors) {
      return this.colors[key];
    }

    return this.colors['C'];
  }
Returns the hex code of a color associated with a key from the current theme.

@param {String} key The color key in the theme (e.g. C, N, BACKGROUND, ...).
@returns {String} A color hex value.