Link to CSS vs HTTPS

I installed the wordpress on january 2018. Since then I installed all updates. The below issue occurs since the installation, even if I installed all updates. But it occurs just spontanously. It disapeares on the next day and comes back after few days.
The <link tag sometimes contains simple HTTP links and sometimes HTTPS. I was not able to find any coherency when there is HTTP and when HTTPS.
See the below example from http://www.opat.sk/category/pivo-a-vino/:

<link rel='stylesheet' id='foobox-noie7-min-css'  href='http://www.opat.sk/wp-content/plugins/foobox-image-lightbox/free/css/foobox.noie7.min.css?ver=2.5.2' type='text/css' media='all' />
<link rel='stylesheet' id='foogallery-core-css'  href='https://www.opat.sk/wp-content/plugins/foogallery/extensions/default-templates/shared/css/foogallery.min.css?ver=1.4.30' type='text/css' media='all' />
<link rel='stylesheet' id='catchkathmandu-style-css'  href='http://www.opat.sk/wp-content/themes/catch-kathmandu/style.css?ver=4.9.6' type='text/css' media='all' />

As you can see, the second line is HTTPS while the rest is HTTP. This happens just sometimes and just on some subpages of my site. For example at the same moment I tried another subpage where it was only HTTP on the same part of the code. http://www.opat.sk/fotky-apartmanu/

<link rel='stylesheet' id='foobox-noie7-min-css'  href='http://www.opat.sk/wp-content/plugins/foobox-image-lightbox/free/css/foobox.noie7.min.css?ver=2.5.2' type='text/css' media='all' />
<link rel='stylesheet' id='foogallery-core-css'  href='http://www.opat.sk/wp-content/plugins/foogallery/extensions/default-templates/shared/css/foogallery.min.css?ver=1.4.30' type='text/css' media='all' />
<link rel='stylesheet' id='catchkathmandu-style-css'  href='http://www.opat.sk/wp-content/themes/catch-kathmandu/style.css?ver=4.9.6' type='text/css' media='all' />

This is not related just to foobox nor to foogallery as I noticed the same begavior also with the theme CSS.
Simpy said, some CSS are linked with HTTP and some with HTTPS and it is not depended on one plugin.
Any idea why this happens and how to prevent it?
Thank you.

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