[tlrobinson]

Are you sure? I think you can edit the text of stylesheets, and add new style properties on individual elements too.

[mhansen]

Editing the text of stylesheets: no.

Editing style rules applied to elements: yes, you can do this. Select an element in the inspector, and you can click to edit its style rules.

[ddemchuk]

unless you're seeing something I'm not seeing, you can only modify existing attributes. If you could show me though, an upvote will be in your future :-)

[Huppie]

Though not as useful as firebug you are able to add a 'style="<custom css>"' attribute to an element by hovering over and clicking the '?=""' that appears.