css = (strings, ...values) => {
    const cssText = values.reduce((acc, v, idx) => acc + textFromCSSResult(v) + strings[idx + 1], strings[0]);
    return new CSSResult(cssText, constructionToken);
}
Template tag which which can be used with LitElement's `style` property to
set element styles. For security reasons, only literal string values may be
used. To incorporate non-literal values `unsafeCSS` may be used inside a
template string part.