What do I need to do the title bar in Opera on Linux? I am using Gnome 3 / GTK in this case.
Please note that I really mean the title bar, not the menu bar or tab bar.
Right-click on the tab bar (yes, I know you want to change the title bar, bear with me) and de-select the option "Show Border". (Maximise the window to see the full effect.) Also see this thread on the Opera forums.
Alternatively, depending on your flavour of Linux (not sure about GTK), your OS may allow you to achieve the same effect by using specific window decoration settings, or you could use Compiz to hide the window decorations altogether (the result may not be to everyone's taste).