I activated my ssl certificate and adjusted most of the links to https:// instead of http://.
But I'm still getting an error in Chrome stating there's "mixed content" loaded:
jquery.js?ver=1.12.4:4 Mixed Content: The page at 'https://www.toonafish.nl/wordpress/' was loaded over HTTPS, but requested an insecure image 'http://demo.qodeinteractive.com/simplicity/wp-content
/uploads/2013/08/home_first_slide.png'. This content should also be served over HTTPS.
It seems like some images are still loading from the Qode demo instead of my website.
How and where can I adjust the URL ?
That is because theme options are saving paths to files with absolute URL which is serialized (number of characters is counted), so you need to replace URL's in the database options table also while serializing input and easiest way to do this is to use WP migrate DB plugin https://wordpress.org/plugins/wp-migrate-db/
Alternatively you could try using some SSL links fixer plugins like maybe following ones https://wordpress.org/plugins/wordpress-https/ http://ithemes.com/codex/downloads/fix-ssl-non-ssl-links.zip but that doesn't guarantee fixing the issue.