This extension gives you the power you once had with the old Presto-based (and even older) Opera versions: To ”turn off“ CSS style sheets (that is, tell Opera not to apply them).
Use the toolbar icon to toggle CSS on and off* or use the keyboard shortcut [Alt]+[G]** for the same purpose.
★! NOTE: Extensions like this never work on internal pages (settings, speeddial, …) or some special pages like the entire „Opera add on“ site here on addons.opera.com !★
* - note, the icon does not show you the state of CSS (yet)!
** – the classic keyboard shortcut is not available in Opera 15+ at this time