When I using a custom theme and editing the font-size of my Navigation component, the sub item text appeared larger than the main parent item text, with no direct way to target the sub-item.
Solution: Don’t use relative sizing, e.g. 1.3em on your Navigation, make sure to use absolute sizing, e.g. 15px, because otherwise it seems that the sub item gets 1.3x larger than the parent item.
