I'm a massive fan of Opera, but I can't figure how to flip the GUI back to where the main titlebar is hidden again. Any ideas?

Back to its original look:

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Under Appearance - Skins, choose "Opera Standard". | |||||
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Unselect "File|Show Menu Bar". If you don't want to see the tab preview, drag and drop the tab line up:
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(I hope I'm understanding your question correctly.) Press Shift-F12, then change the appropriate checkboxes under the "Toolbars" tab to suit your needs:
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