Ok, it seems my message is unclear. Let’s try again.
Your theme contains links to sites: schema.org and placehold.it
Both sites are HTTPS, not HTTP. And the theme links to HTTP versions. As an example /lib/custom.php line 149:
$schema = ‘https://schema.org/’;
And if in a case of schema.org it`s just a good practice to replace HTTP with HTTPS, in case of placehold.it it breaks security because of unsafe image load, which is bad.
Hope I explained better now.