Responsive Site Top Margin

Can anyone tell me how to add a top margin to a responsive site. Using the following code:
<style>
#xr_xr {
margin-top: 15px;
}
</style>

This code works fine when inserted into the head tag for a non responsive site, but will not work for a responsive site.
I have been told to use the <element> tag instead of the <style> tag but I am unsure is this is correct or how to use it
Can anyone help me please?
many thanks in advance

I have moved this topic to the Web Design Forum.

This seems to work (only gave it a brief test, let us know how you get on)

Code:

---

<style type="text/css">
div.xr_ap {
margin-top: 15px;
}
</style>

---

Thanks for your quick response.....and yes it appears to work fine.....