Because you don't have resources for xxhdpi
for Action Mode. Use Android Action Bar Styles Generator, generate resources for your Action Bar
in the action mode
and apply it. It will fix your problem.
Pregunta
I have a light gray background for the contextual action bar. But on a Nexus 5 I have also a darker overlay and I don't know why :-(
I think the images for the background are okay.
Solución
Because you don't have resources for xxhdpi
for Action Mode. Use Android Action Bar Styles Generator, generate resources for your Action Bar
in the action mode
and apply it. It will fix your problem.