[Merge] ~3v1n0/ubuntu/+source/gnome-shell:ubuntu/bionic-theming-fixes into ~ubuntu-desktop/ubuntu/+source/gnome-shell:ubuntu/bionic

[Didier Roche]

Review: Approve

Please ensure the bugs are SRU-able. LGTM otherwise.
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https://code.launchpad.net/~3v1n0/ubuntu/+source/gnome-shell/+git/gnome-shell/+merge/356023
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