Window list theme is not consistent with the desktop

Since we switched to standard black shell theme insted of GNOME Classic grey, the theme of the windows list in the bottom panel doesn’t look coherent with the rest of the desktop. The “button” look neither like the rest of the shell nor like GTK3’s Adwaita default theme.

Related issues

Related to #12415

Original created by @alant on 12233 (Redmine)

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