Hello,

Thank you very much for choosing our theme and contacting our support.

SSL certificate is the settings configured with your hosting provider and our theme does not influence it. If you switch to a default theme you will see the same warnings. When you delete demo images and content and upload yours and these warning would disappear cause your content would be uploaded via HTPS. Please find more information here: https://managewp.com/blog/wordpress-ssl-settings-and-how-to-resolve-mixed-content-warnings

If you have any question, please feel free to contact us.

Best Regards