The current ICON_SMALL_TRI_RIGHT_VEC uses dark hard-coded colors ([0.2, 0.2, 0.2])
which makes it impossible to theme and hard to see in dark contexts.
Use ICON_RIGHTARROW to match the Outliner's breadcrumbs. This icon uses TH_TEXT so it's visible as long as the rest of the text is.
Master
(Properties editor background made red on purpose to be able to see the triangle icon)

