I'd be grateful if anyone has the answer to this. I have the flex theme by out of the sandbox and it allows you to enter sections where you can add custom HTML.
I created a new section so I can add the Klaviyo newsletter sign up form- near the bottom of the page. However I'm unable to increase the size of the email entry field, despite me changing it in the Klaviyo app. I suspect it's something to do with the div class restricting it but I'm not sure how to fix it.