>> implement a setting to adjust
Sorry if I failed to explain but...
Google's designers changed the styling of notifications in Android 5.0.
They are now: dark text on light background.
The change is intentional, "it's more elegant, subtle, yet intuitive, and easy to read" (or some such, I'm making it up, but I'm sure they had a reason).
Changed in Android.
In Android.
Android.
Android 5.0.
Changed.
By Google.
Intentionally.
Because they think it's better.
Feel free to file a bug report / request at
http://b.android.com (which will almost certainly be closed with "working as intended", but it never hurts to try).