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": https://git.libreoffice.org/core/commit/b3d62dd95446a7289ddcc33697811e7acefe4057 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: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
that looks about right to me