GNOS Project (https://gnos.in) relies on LO thumbnails (generated by LO and zipped at Thumbnails/thumbnail.png) to generate odt/odc/odp files previews.
Since version 6.3, thumbnails get a black background (instead of the white one present in 6.2 and previous versions) rendering most file previews unusable.
# Steps to reproduce
- Install LO 6.3.2 on Ubuntu from ppa:libreoffice/ppa
- Create any document, save
- Check Thumbnails/thumbnail.png with any image viewer
- Expected behaviour: the preview should have a white background
- Actual behaviour: the preview has a black background
I believe you're using gtk2 or gen. I believe it's a duplicate of bug 125461.
It's fixed in the upcoming version LibreOffice 6.3.3
*** This bug has been marked as a duplicate of bug 125461 ***