That's because it means that the <body> tag in your style is replaced with something else usually. To get around this, just add a <body> tag underneath this one - in other words, have just 1 <body> and then have the code as well.
If you just want to have 1 body tag, copy the body tag contents from your style, and paste it into the body tag in the template. But this is time consuming, as you would have to do this for each style that you have.
So in essence, I take back what I said -

- to maintain your background colors, you need to have <body> still in your privmsg, as well as my additional one.