We've ported the SDDM theme to PC3 and now have black labels in PC3 ToolButtons (keyboard and session button).
The SDDM theme uses a complementary color scheme, which is something the PC2 ToolButton respects and turns the labels white.
Therefore I just copy pasted PC2's label color code to its PC3 counterpart.
BUG: 414929
FIXED-IN: 5.66