shapeStyle = (shape) => {
    const selector = `.${CLASS_NAME_PREFEX + shape}`;
    const rule = `{
    border-radius: ${shape === 'rect' ? '0' : '50%'};
  }`;
    if (!styleCache.has(selector)) {
      styleCache.set(selector, rule);
    }
  }
a Map instance to cache the styles which will be inserted into the skeleton page.
key is the selector and value is the css rules.