This is a test file ONLY for the problem described in post
#21.
I am changing the class name (in the CSS file) from ".red" to ".warning".
It turns out that this problem is even older (Update Preview Window also does not work then), because I see it in versions 2.5.2, 2.6.0, 2.6.2...
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So it's possible that these are three different issues.
* One described by @Turtle91.
* The second related to ignoring custom_preview_style.css.
* The third related to the Preview window not responding after changing the class name in the CSS file.