The background color on the jotform thank you page does not work; it is transparent

this is a big problem, how might i fix this?

You can setup the background to whatever color you wish in Thank you message editor

 Just click the HTML icon and manually enter some HTML that will get you somewhere close to this:

 e.g

<div style="width: 100%; height: 100%; background: red;">
<p>&nbsp;</p>
<div style="text-align: center;">
<h1>Thank You!</h1>
Your submission has been received.</div>
</div>

See my form here

Thank you for the reply! However this html code you've provided will only change the header color.

There is a built in background color selector in the thank you page editor but it does not work.

The rest of the thank you page remains transparent

I am using my form as an iframe

Yes, the editor of the thank you message does not work very well. What I do recommend you is to make an HTML page and then set a Custom Thank page (URL) so that the user will get redirected to it.