Setting up your site:
“Can I personalize the colors of the background on my site?”
Yep yep yep. You’ll do this under Appearance on your dashboard. There’s also controls for changing the background, etc., in the products page.
“My site is set to add a flat rate shipping. I want the cart to figure it out by weight. Where can I find that setting?”
You can change that option under Products–Store Settings–Shipping. Be sure to use the weight of the package as it’s ready to be put in the mailbox. A good way to do this is to package the item as if you’ve actually sold it and then weigh it. That’s the weight you’ll use on the item’s entry in your product list.