Revert "Explicit set header text colors"

This reverts commit 4fd22ca4d6.

app/src/main/res/layout/item_group.xml

@@ -12,7 +12,6 @@
         android:padding="6dp"
         android:text="Category"
         android:textAppearance="@style/TextAppearance.AppCompat.Medium"
-        android:textColor="?android:attr/textColorSecondary"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="parent"
         app:layout_constraintTop_toTopOf="parent" />
@@ -25,7 +24,6 @@
         android:padding="6dp"
         android:text="Today"
         android:textAppearance="@style/TextAppearance.AppCompat.Small"
-        android:textColor="?android:attr/textColorTertiary"
         app:layout_constraintEnd_toEndOf="parent"
         app:layout_constraintStart_toStartOf="parent"
         app:layout_constraintTop_toBottomOf="@+id/tvCategory" />