The SSLSSL Secure Socket Layer - Encryption from the server to the browser and back. Prevents prying eyes from seeing what you are sending between your browser and the server. certificate is broken on the wp-themes.com domain, or rather, it is using the certificate for *.wordpress.orgWordPress.org The community site where WordPress code is created and shared by the users. This is where you can download the source code for WordPress core, plugins and themes as well as the central location for community conversations and organization. https://wordpress.org/. This causes issues with the theme previewer for assorted reasons.
Any chance of getting that certificate corrected?
Related: https://meta.trac.wordpress.org/ticket/483