Design tweak: reading list onboarding button style.
Design tweak: reading list onboarding button style.

Change-Id: If4ba9f125fdaaf2f8de44debabc3f8ee9b3e59b7

dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia,dbrant/apps-android-wikipedia,wikimedia/apps-android-wikipedia