Sorry to ask, but WFM means "works for me", right?
Yes, sorry.
(see code in the report),
Used copy and paste of that CSS but with 30px just to make sure its obvious.
I use Geany 2.0 with this info: GTK 3.24.41, GLib 2.80.0.
Well, its Geany Git version, and slightly older GTK/Glib. Remember the sidebar is standard GTK only, nothing special so it should work for your newer versions unless GTK broke.
Just to check, you did close and restart Geany each change of the CSS.