The stylesheet menu item allows you to edit your local CSS stylesheets (i.e. the stylesheets pertaining to your website alone).
The editor opens on the stylesheet defined as the local stylesheet in the Layout menu item, but you can use the buttons at the bottom of the page to open, save, close any stylesheet in your local styles directory.
CSS (Cascading Style Sheets) is a tricky area, and there are subtle differences in how different browsers render the same styled page. If you are not versed in CSS I would recommend that you read one of the standard introductions to the topic (see for example what's on offer on Amazon). Also check out the CSS Web Tutorial on W3Schools.
For a review of the fundamentals of CSS I would recommend 'CSS The Definitive Guide' by Eric A. Meyer (O'Reilly). This is definately not a beginners book, nor a particularly easy read, but indispensible for understanding the quirks of the language.
To be able to style the page sensibly, you'll need to understand the structure of the page. This is best achieved by using a developer tool such as Firebug in Firefox.
It may help to know the philosophy behind a Cradur page structure. This is described in the following page:
If you are using Internet Explorer, the editor is a simple text editor, which is primarily intended to allow you to make adjustments to your CSS styles. It can of course be used to create large style sheets, but I would recommend you using a dedicated style sheet editor for this, and importing either by FDT'ing the file (see your systems administrator for this) or by cut-and-pasting into this editor.
For other browsers the Ajax.org Cloud9 Editor (ACE) is used. This has the additional advantages of line numbering, syntaxt highlighting and checking, though the warnings and errors can be a bit 'buggy'.