ActionMode Styling and Reflection

I am using reflection to change the background of my ActionMode like the snippet here:

https://stackoverflow.com/a/43489925/13345844

It works but after building the project, and using the release, it stops working. I am assuming this is caused by ProGuard but I have not worked with reflection before, so I do not know what exclusions I need to add to my proguard file.

Can someone point me in the right direction, please?

Thanks.