Fix body text color in mono theme

Because it's either white or black let's use color that contrasts better with the background.

Bug: 369985720
Test: manual, turn on mono theme and see text is visible when switching between dark and light theme
Flag: com.android.systemui.shared.new_touchpad_gestures_tutorial
Change-Id: I04cc67ae631d1937e6715bc323f47daba5a79154