Found the problem. The extension User-Agent Switcher was turned on.

My thanks to all that replied.

YOUR BROWSER SCORES 501 OUT OF 555 POINTS
You are using Opera 48.0 on Windows 10

I am on Version 58.

This is using http://html5test.com. I am on Opera version 58.0.3135.79.

What is wrong here?

For topics that have many posts, there is a "counter" at the bottom of the page. I know this is trivial, but is there any way of disabling this?

@pastel10 ... Try this ..., in Settings, Enable Dark Theme. (which you have done).

Then under wallpapers, select a black wallpaper. That should work.

On my setup, the active tab is a dark gray background with white text.

The inactive tabs are black with white text. I also have minor vision problems and my setup makes it
much easier to read. Hope this helps.

One other thought, disable any tab extensions that might conflict with this.

What helped me was to use a dark theme so that the tabs are white text on a black background.

I just stumbled upon a new amazing bookmarks sidebar extension in the Chrome extensions library.

It's called Bookmark Sidebar and allows the sidebar to pop-up using the cursor only. It also allows quite a few options for customizing the actions, font sizes etc. Very impressive.

@leocg ... I hope my following reply does not come across as snarky.

It's time Opera got back to basics. If the Sync procedure does not work as it should, then why enable it at all? Fix it so that it works properly.

Does the average user really need "smarter news" and Netflix
recommendations? (From the blog). Same with crypto wallet access.

I'm still waiting, (not so patiently), for a proper bookmarks sidebar. Similar to Firefox, that can be accomplished there by using prepared CSS code. Yes, I know Opera has sidebar extensions, but they are clunky.

i hope the near future will see Opera return to its roots.

@rif ... Does this not change the Opera "statistics" on web browsing? Is it a good thing or a bad thing?

I am using https://blogs.opera.com/desktop/. And I see now there are new entries.

I guess, perhaps, I am just too impatient. My apologies. Thanks to all that replied.