After modifying stylesheet.css you need to go to ACP, Styles, twilightBB and clear the theme. The same applies to when you modify the size of any forum icons, in which case you'll need to modify the appropriate cfg file, and then reset the imageset from the ACP.
I am having the same problem with the banner/header.
I don't understand this.
I am a co-admin I guess you could say, I didn't create the forum.
Does the initial admin have a "styles" tab in the ACP?
Do I have to get him to "clear the theme"?
Once the theme is cleared will the banner re-size or will I have to mess with cfg (theme.cfg)?
Is "theme.cfg" the "appropriate cfg file"?
What do you mean by resetting the imageset?