How to change background color?

Open the file style.css in your theme directory and add a rule to change the colour of the background. Looking at your code, you would need to replace:

body {
  font-family: Helvetica, Arial, sans-serif;
}

with…

body {
  font-family: Helvetica, Arial, sans-serif;
  background-color: #000000;
}

#000000 is the hex representation of “black.” Look at this chart to find a colour you like.

You would also need to remove (or replace) this line in your other header.php template file.

<style type="text/css">
body { background-color: #FFFFFF; }
</style>

Changing this would invalidate the need to perform the instructions I posted above.