Okay, here you go.
First, make a picture (if you're wanting a black background, open an art program, make it all black, save it. That easy), and make it what you want as the background. Then upload it, yadda yadda yadda (pretty sure you already know how to do that) and put this on the pages you want the background in. Make sure you have the RTE turned off.
<html><style>body{background: url('Insert Url of picture') fixed 50% / cover;}</style></html>
Put this for where you want to change the color to white. (black would be all '0' instead of '255')
<p><span style="color:rgb(255, 255, 255)">Put text here.</span></p>
