如何更改HTML框角的颜色？

Question

 <iframe width="100%" height="500" src="https://minnit.chat/Real97ChatRoom?embed&amp;web" style="border: none;" allowtransparency="true"></iframe><br /><a href="https://minnit.chat/Real97ChatRoom" target="_blank" rel="noopener noreferrer"></a> 

I'm trying to put a chat room on my website and I was given a embedded code in order to do this but for some reason the corners of the box are white and it looks unprofessional. I believe the reason why it's showing a white corner is because the chat room box itself has rounded corners and the background possibly isn't transparent? So it's leaving the white mark. I'm not too sure but I want to see if there's a way to add some code to turn those white marks black or transparent so I can keep the rounded corners as is. I'll leave a photo below as reference. Please open the picture fully to see the white marks I'm talking about.

The code I was given is listed above as well.

Take a look and see if you can help.

Thank you.

Answer 1

<style>
iframe{
    -webkit-box-shadow:inset 0px 0px 0px 10px #f00;
    -moz-box-shadow:inset 0px 0px 0px 10px #f00;
    box-shadow:inset 0px 0px 0px 10px #f00;
}
</style>

You can try this code it should work fine.