This is just enough to draw the sidebar background and markt he current page as active.
975f29f Adwaita: Add assistant sidebar theming
gtk/resources/theme/Adwaita/_common.scss | 13 +++++++++++++
gtk/resources/theme/Adwaita/gtk-contained-dark.css | 7 +++++++
gtk/resources/theme/Adwaita/gtk-contained.css | 7 +++++++
3 files changed, 27 insertions(+)
Upstream: git.gnome.org