Created attachment 119770 [details]
On GNOME with GTK/Adwaita, the LibreOffice UI toolkit displays its "infobar"-style widgets (such as the one when a document is opened in read-only mode) with a gray background. It ain't pretty.
Thanks for your report. This is already fixed towards 5.0.4.
*** This bug has been marked as a duplicate of bug 92217 ***