Look the image above, the tooltip shows a preview of contents in the text/plain file in a white background, the content-text color is also white; therefore it is useless.
I am using a dark theme and dark themes should use white for normal text.
This problem is not happening in light themes.
How to fix this issue ?

Which exact theme and Plasma version are you using? I currently use Breeze Dark on Plasma 5.17.5, no problem with the tooltips here.

Mamarok wrote:Which exact theme and Plasma version are you using? I currently use Breeze Dark on Plasma 5.17.5, no problem with the tooltips here.

OK, thanks. I noticed that it happens for .txt files only, even in Breeze Dark; others (like .html, .xml, .js etc) are OK.

From your screenshot I guess it shows a blank sheet for a text file then? In that case I guess that's how it should be

FWIW: taking screenshots with spectacle can be done on a defined portion of the screen, no need to do a full screen snapshot

In your screenshot, the file size is just 1 byte, so practically empty, which would be why nothing appears in the screenshot.
Can you try editing the text file, adding some more text, save and then try the preview again? I would expect that if there were any text in the document that it should show up in black-on-white, just like the other formats you mentioned (which are all just different flavors of text file and ultimately should be using the same preview style).