The map is inside an iframe, and unless I’m missing something here, there is no way to style content in an iframe which pulls content from a third-party site.
Just a thought, but wouldn’t inline styles inside the <iframe> tag work? I believe I did this once on a site that was pulling in suggestions of book titles from Amazon.
No unfortunately you can’t access content in an iframe unless you have access to both iframes (on the same domain) and then with js use post message to communicate between the two. Both must be set up with scripts to send and receive. Which in most cases is irrelevant as you could then just style the iframe content at source anyway.
An iframe is just a window to another site and you can’t style other peoples sites.
I think that’s just for embedded maps in an iframe.
If you use the API then I believe you can build your own header into it. However, I have never tried this so don;t know how feasible it is unfortunately.
Probably too late to be of help to @BrianBam, but I’ve just discovered https://www.openstreetmap.org. They apparently place no restrictions on what you can do with their maps - and their embedded maps have no header in any colour.