PageSpeed recommend “rel=preload”

Hi,
I am using your nice theme.

In the PageSpeed Google Insights for https://hotelesliguria.com/ I see this:
“These optimizations can speed up your page load.
Consider using <link rel=preload> to prioritize fetching resources that are currently requested later in page load.
/customizr-pro/assets/front/css/magnific-popup.min.css
/customizr-pro/assets/front/css/flickity.min.css

How can I do something like that? How fix “preload” in there two css (magnific-popup.min.css and flickity.min.css? Or other solution/suggestion?

Thanks for help!
Albert

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