Indulge An Old Guy, If You Would Please (Phone Assistance)

Near the end of last night's LSU basketball game, several students held up their phones with messages such as "Get Loud" in very large, easily legible, letters. (Or at least they were large for a cell phone.) In particular, the phones I saw had black letters on a yellow background.

How is this accomplished? Is there an app that does this? Can it be done using your phone's settings? Thanks for any info someone may be able to provide.