go-playground: set code background color explictly
go-playground: set code background color explictly

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/100020044

Alexandr-Penkin/go-playground,Alexandr-Penkin/go-playground,stevelacy/go-playground,stevelacy/go-playground