Display subcategories with some left-margin?

The best way to do anything like this is to look at the source, or even better, the DOM-inspector in Firefox. The DOM-inspector allows you to point at an element and see the structure of the nested classes/id’s. In your case, a child will probably look like this:

#sidebar ul li ul li ul li

meaning sidebar -> list (all sidebar sections) -> list-item (one sidebar section) -> ul (categories) -> li (category) -> ul (subcategories) -> li (subcategory).

In style.css, give this tag a small left margin:
`#sidebar ul li ul li ul li {
margin-left : 1em;
}