Anytime you see a line that starts with a .something or #something followed by something in curly braces, it goes in the stylesheet (in your case, includes/templates/jewelry_store/css/stylesheet.css. Usually, you can add stuff anywhere in the stylesheet. I put the things I add at the end so I'll know what I added.
You need to install the Firefox browser and the Web Developer plugin for Firefox. You can use those to see where in the stylesheet you change the styling of the various sideboxes.
Ctrl-Shift-Y will allow you to hover over a section of the page and see the class and id names for that section in the box at the top. The statements in the stylesheet that begin with # are id's and the ones that begin with dots are classes.
Ctrl-Shift-E will open the CSS editor and allow you to edit the stylesheet and see the changes in real time without changing anything permanently. A very handy tool.
With most sideboxes in Zencart, the id with a 'Heading' after it will style the sidebox header. (Note:CSS is case-sensitive.) So, once I found (using Web Developer) that the shopping cart sidebox id is #shoppingcart, I added Heading to it and colored it red with that statement in my previous post. The information box would be #informationHeading.
Most important thing is to not let this stuff intimidate you. It's mostly a matter of just messing around and trying things.