There are basically three issues here:
- the renderer doesn't add the GTK_STYLE_CLASS_EXPANDER style class when
rendering, which blocks the theme to apply the intended colors.
- the GtkStateFlags that were set on the context were using GTK_STATE_*
instead of GTK_STATE_FLAG_*
- the flags set on the context were incomplete, as they were just using
ACTIVE/NORMAL for expanded/collapsed and not SELECTED/FOCUSED/...