Yes you can edit the background on your site by simply editing your site's CSS file.

Find the file inside the "css" folder - it's called "common.css"

Open the above file with a text editor (eg notepad) or a CSS editor and at the top you will see:

body {
background-color: #E3E3E3;

You can change the colour of your site background by changing that #E3E3E3 hex code to a colour of your choice for instance #FFFFFF is white and #000000 is black

All available colours can be found here:
http://www.webmonkey.com/reference/Color_Charts

If you wish you can also set the background with an image

to do this replace

background-color: #E3E3E3;

with

background-image: url(http://www.yoursite.com/images/background.jpg);

where yoursite is your actual site and images is the folder where the image resides.  background.jpg is just an example name (your image can be called anything you like)

Now all you have to do is copy and replace your newly edited css file back to your Instant Member installation folder on your hosting account, refresh your site in your browser and you will see the changes instantly.

Note:  If you haven't edited a css file before we recommend you first take a back up copy in case you wish to revert back to the default settings at a later point.