Custom Google Search causing mixed content

I have google custom search installed and it works ok.
I have recently updated my site to use SSL and this has resulted in my site getting mixed content.
I understand that all HTTP needs to be changed to https.
My problem is I am having an issue with custom google search loading its stylesheet over HTTP and not https.
All other plugin stylesheets have adapted nicely, but not for custom google search.
I can tell that the error is from the linked stylesheet for the index.html that is loaded. See screenshot.
All I want to know where can I go to edit this file in my file manager or PHPMyAdmin.
All assistance is welcome.