The "dark mode" does not work properly.

The text becomes light, the logo changes, but the background of the page remains white.

I set the background colour of dark mode to black.

The other problem is that the top bar was visible for a short time after I set it, but then it disappeared and does not appear even though I enabled it.

Thank you. 

I have disabled the cache plugin, but the situation has not improved.

The biggest problem is that when visitors load the page, they see light grey letters on a white background which is unreadable. They have to turn off the dark mode to be able to read, but I don't think anyone can figure that out as the page background is white.

Unfortunately, dark mode is the default, even though it is not turned on.

A very strange thing happened. I turned on the "Set Dark Mode as Default" switch, and then turned it off immediately afterwards. Now it is off. In Firefox browser, the background is black, half the menu is white, the other half is grey. In Chrome, the background is sometimes white sometimes black, and the menu is blue. 

The default dark mode is still off.

I replaced the cache plugin, but no result.

I found the error.

There is a hint at the top of the Dark mode options:
"The default theme must be used in light mode for it to work perfectly."

In color options you have to turn off the "Enable Dark Theme".

Now it works.