There are margins in the standard templates - e.g., below the navbar and between top/content sections.

Generally speaking this is good, because we don't want content touching the navbar. But in the case of using the Background includelet it does mean you're left with a gap below the navbar.

One sneaky fix for this is to give the background includelet a -20px margin (i.e., negative 20px). This will mean the background closes the 20px gap below the navbar.

You might then want to add a 20px padding at the same time, so that the contents of the coloured area aren't too close to the top.

As for adding a colour to the entire page - there is no theme option to do this, but a bit of custom CSS is easy to add. e.g., in the Raw Header HTML in the theme options you could add something like:

<style>
body {
  background-color: #f7ffec;
}
</style>

Hope this helps!

Joe