PULL_REQUEST_TEMPLATE: add "UI change has been tested on both light and dark themes"
PULL_REQUEST_TEMPLATE: add "UI change has been tested on both light and dark themes"

vector-im/riot-ios,vector-im/riot-ios,vector-im/riot-ios,vector-im/riot-ios,vector-im/vector-ios,vector-im/riot-ios,vector-im/riot-ios,vector-im/vector-ios,vector-im/vector-ios