Change font of autogenerated email

Question

Hi Folks,
I have another question and because of points being granted for the correct answers, I did not include in the previous post.
When  any users register on a site, we send them an email to activate the account  and verify their email from the server. I would like to change the font  of that email.
What determines the font or where can I make that change?
Thank you,
zabberwan

Answer 1

Thanks JS,
I do already have that included:
//headers info (simple for the moment, shall change later to verbose)
            $headers = "From: $adminEmail\r\n";
            $headers.= "Content-Type: text/html; charset=ISO-8859-1";
            $headers .= "MIME-Version: 1.0 ";
By "using" CSS do you mean I append or link a CSS file?
@ John Waller: I did not post the code, as I'd rather not copy and post the code on my remote server and expose it to all forms of mischief. It was done on purpose.When I said it was functioning properly, then it is obvious that the code is generated with the header info pasted above. Otherwise it would not function. I suppose you know that.
I had forgotten to mention it was PHP which I did on being asked for it.
Thank you
zabberwan