Well, In the example, libreoffice calc file, ods.
Do you notice the contrast with the surrounding background color? exactly!
Why dolphin want this dazzling high contrast preview?
I feel that users who have kde plasma theme in a dark shade that is gentle on the eyes are under attack! can change preview color?
I don't know the site for attaching screenshots. If it can be found, it can be attached as an example.

https://imgur.com/RvoTMjJ
its preview. can you see contrast? and feel eye attack? exactly. i don't want people to damage their eyes in the image preview.
That's why I decided to upload in link format instead of thumbnails.