Good morning Stefaan and welcome back!
Thank you for reaching out to us.
We are sorry to hear about that extra border coming from the iFrame.
I would love to help you with this topic.
I took a look at your website and it turns out that there is no CSS rule define for borders of anĀ iframe in your theme.
As a result, it defaults to your browser user agent stylesheet. CSS rules on the iframe’s main container will work as long as they are specified before loading the page.
You have a couple of options available, but here’s one that should do the trick:
iframe { border: none !important}
Another option would be to make the border-width 0px or to make it white.
Let me know how that goes.
Best regards,
Geoff B.