Hi Mike
I don't use easy member pro but ...
This sounds like a css issue instead of a html issue since you say you're having a problem with the backgound. CSS handles display (colors, etc), the html handles the layout (page width, etc). I'll try to answer your question about CSS.
You basically have two options for css. If it is only a few lines, you can put it on the webpage itself between <style type="text/css"> your code </style> in the header.
If you want to load it in your webhosting account, you would put the css file in the same directory as your webpage - your root directory or what ever directory your webpage is in. You will need <style type="text/css"> <link rel="stylesheet" type="text/css" href="yourcssfilename.css"> </style> in the header of any webpage that you want to read the file. You can FTP the CSS file to you hosting account or use your cpanel's file manager to place the file in the same directory as your webpage.
I hope this helps - George