Hi,
Do you have any knowload of CSS and HTML at least? it would help a lot, otherwise even if anyone tells you how to do this you woun't understand what you're doing and might end up changing something you shouldn't change!
Anyway the background is easy to change, and you have two options to do it!
Open the skin.css flie located in the skin folder (should be Portals/_default/Skins/MinimalExtropy/) and just in the top (line 4) you find this:
#Body {background:#FFF url(images/body_bg.png) repeat-x top left; height:100%; margin:0; padding:0; }
Has you can see an image file is beeing used for your background, you can either change this image (body_bg.png) to a white image, or change the css whish I believe would be easier for you, so you should change it to this:
#Body {bacground:#FFFFF; height:100%; margin:0; padding:0;}
Now all the other stuff can be cahnged do you you'll need to see the skin files has they are different in each version...
DON'T forget to make a copy of the file you will cahnge, so you can replace it in case you do this wrong!