That was one of the primary reasons I used Opera.

That and the "hide images" button (it worked instantaneously, without the need of reloading the page), also the "invert colours" option, also the zoom (this was way before Opera 12 though, at that time other browsers didn't even have the zoom function).

However, you could try Vivaldi, it has the same skin layout of the old Opera: in the bottom bar (right beside the "hide/show images" button) there's a button with which you can do some CSS customization like invert colours, greyscale, sepia, (there isn't a "fit to width", but the program is still in beta).

It also seems pointless in the modern web with the advent of responsive design.

Sure, responsive design works flawlessly, the usual result that I get when zooming in is that menu elements or other boxes get so bloated that they cover the text below, at that point I'm forced to reduce the zoom to being able to actually read the site.

After some time I've restarted using Opera (I used it since version 4 or something, then I stopped when it was "chromefied" and now I'm back ).

I have imported my bookmarks from other browsers and now I have 20 or so folders in my "Imported" list.
When I open "Imported" from the bookmarks bar, the folders fit with the screen height without the need of scrolling (desktop resolution is 1920x1080), but when I click the "heart" icon to add a site to my bookmarks, I cannot place it to one of my lowest folders because I can see only half of them, the other half is cut away below the screen. is this a bug or there's something wrong with my installation?

Is there someone with the same problem?