in the developer toolbar, select Outline -> Outline current element, if you now hover over an element the name of the element appears in the status bar, this is sometimes too long, use the DOM inspecter then
to edit css, in the developer toolbar click CSS -> edit CSS, then select the rsgallery.css tab and edit away and see live results, after that copy and past

you can edit source code with the SourceEditor, goto (in the firefox menubar) Extra -> Start - Stop SourceEditor, then double click on the element you want to edit and there you go