I am using caldroid library for calendar activity and It works fine.
But I needed to change it's style.
So I added caldroid library(directory) under the application like this.

and I changed my dependencies
dependencies { ... compile 'com.roomorama:caldroid:2.3.1' ... }
to
dependencies { ... compile project(':libraries:caldroid') ... }
Then, It works fine under the debug mode.
But I can't generate signed APK because of this message.
Error:Execution failed for task ':app:validateExternalOverrideSigning'. Keystore file /Users/xxxxxxx.android.keystore not found for signing config 'externalOverride'.
Is there any solution about this error?