If you change anything in the tree (added resources or relations, new or deleted pages, or moving of pages), you must remember to click the "save" icon in the toolbar.
Editing content is pretty straight forward. Select a menu item in the tree structure, and edit at will in the text editor. Remember to save and compile your work.
To fully edit a style you need to understand CSS, style templates and XSLT templates. Most people should only need to work with the style templates, and use already exsisting CSS and XSLT templates associated with a certain design.