1. A computer-implemented method comprising: receiving, via a network interface, user input from a plurality of computing devices on which respective instances of a program execute, the user input identifying user-defined displayable content for replacing other identified displayable content of the program, the other identified displayable content displayed by the program at one or more of the plurality of computing devices, the user input received from one or more users of the program; assembling the user-defined displayable content into a content update package that corresponds with the identified displayable content and that is suitable for transmission over the network; and transmitting, via the network interface, the content update package to another computing device from among the plurality of computing devices on which another instance of the program executes effective to cause the other instance of the program to implement the content update package to replace display of the identified displayable content with display of the user-defined displayable content at the other computing device.