Created attachment 166860 [details]
gtk3 backend theme
It look like the .uno:StylesPreview (stylespreview.ui) use the wrong widget background color.
its not obvious to me what widget you are referring to in the screenshot ?
Created attachment 166867 [details]
StylesPreview has the wrong widget background
In the notebookbar.ui file there is .uno:StylesPreview which include stylespreview.ui into notebookbar.ui and the stylespreview.ui section didn't use the correct color.
ah yes I see. The stylescontainer is centered vertically inside a GtkBox so the default gray is shown in that unused vertical space.
Caolán McNamara committed a patch related to this issue.
It has been pushed to "master":
tdf#137865 use stack background to match the other notebookbar areas
It will be available in 7.1.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
that looks about right to me