I want to know in general which CSS commands are useful in customizing elements of the sidebar on the home page.
I looked at http://multicoach.wordpress.com/ and I see an area on the right that starts with "If you're looking for expert." Is that the sidebar area you are referring to? Here is a CSS example that can target that specific area:
border: 5px solid red;
It adds a huge red border, so try previewing it only and then adjust it from there.
To find out what selectors to use in CSS, you can use your web browser's built in browser tools. Try right-clicking on an element and then select the "Inspect Element" option. It should pull up a panel with the HTML highlighted for what you right-clicked as well as all the CSS that applies to it on the right.
You might also want to check out a CSS tutorial like this one if you're just getting started: