colorFromSource = (source) => {
    if (source == this.SRC_DEFAULT) {
      return this.DEFAULT_COLOR;
    } else if (typeof source == 'number' && Number.isInteger(source)) {
      return `rgb(var(--hterm-color-${source}))`;
    } else {
      return source.toString();
    }
  }
@param {symbol|string|number} source
@return {symbol|string}