I'm closing this, as Frederik told, there are multiple ways of achieving that, and not that no other way is supported upstream, in GNOME Shell. If you want another way, please open the issue on the GNOME Shell upstream gitlab issue tracker.
You would note that you can do it: - via the user-themes extensions - or switch to the gnome-session, which is the convenient way we (ubuntu) provided for people in your case, wanted to be closer to default GNOME look and feel. We can't support officially (like adding other user's session) for combination of different Shell theme - but with this GTK theme, and another Qt theme). A choice had to be made and we tried to covered most of them, even coping with people who wanted default Shell theme (via the session), which required quite some engineering. I hope you understand. ** Changed in: yaru-theme (Ubuntu) Status: New => Won't Fix -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to yaru-theme in Ubuntu. https://bugs.launchpad.net/bugs/1788547 Title: gnome-shell notifications use Yaru theme even if Yaru is not selected Status in yaru-theme package in Ubuntu: Won't Fix Bug description: Since Yaru became the default theme (https://bugs.launchpad.net/ubuntu/+source/ubuntu- settings/+bug/1783571), gnome-shell's notification boxes (and also the execute command box shown by pressing ALT-F2) are themed using Yaru instead of the selected application theme. For instance, in the attached screenshot, the application theme is Adwaita-dark, but there is a notification box displayed with a white background and dark text. To match the theme, the notification box should have a dark background and light text, as it used to before Yaru became the default theme. ProblemType: Bug DistroRelease: Ubuntu 18.10 Package: yaru-theme-gtk 18.10.2 Uname: Linux 4.18.4-041804-generic x86_64 ApportVersion: 2.20.10-0ubuntu7 Architecture: amd64 CurrentDesktop: ubuntu:GNOME Date: Thu Aug 23 13:58:26 2018 InstallationDate: Installed on 2017-08-16 (371 days ago) InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412) PackageArchitecture: all SourcePackage: yaru-theme UpgradeStatus: Upgraded to cosmic on 2018-08-15 (7 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/yaru-theme/+bug/1788547/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp