It's very hard for an add-on to pin-point what to darken and what not, so most of these night-mode add ons, across all browsers don't look very nice.

To have a very well functioning night-mode add on it needs to be website specific, so you'd probably do better to do the following:

• Look for website enhancing and/or website specific night-mode add ons. (e.g. magic actions for youtube, reddit enhancement suite for reddit)
• Lower your monitor brightness, or use a brightness lowering overlay (e.g. f.lux or dimmer) if you simply want something more easy on your eyes.

Although not really an answer to your question, I hope this post can help you achieve some goal.

A bit more information:

http://prntscr.com/bvnmvc This is what it looks like, it simply looks as if the css was omitted, but clicking on "add to Opera" on any add-on page will not produce any results besides scroll the page to the top.

Hi, I don't mean to hog this thread, but I'm having the same problem, and a screenshot of what you asked.

http://prntscr.com/bvnerh

I have tried disabling all my extensions, without result.

The page displays properly on Chrome and Firefox, just not on Opera.