Think about the frozen pizza category. There's only one brand that stands out and that's DiGiorno. It's not because they have 'rising crust' pizza – every brand has that now. It's not because they were first – if they had just said "now frozen pizza is even better!" they'd be an afterthought. It's not because they had the marketing muscle of Kraft behind them – Jack's, Tombstone and California Pizza Kitchen were all Kraft brands.
It's that they were able to crack a code with consumers in five simple words, "It's not delivery. It's DiGiorno."
With that one phrase, they went from being just another frozen pizza to an acceptable alternative to Domino's, Pizza Hut and other delivery/take out pizza brands.
Those five words informed everything they did: their toppings, their packaging, and their marketing. It's a rare theme line that provides not just a clever hook for consumers, but a mission statement for employees. It stands for something not just functional (a better frozen pizza) but aspirational as well.
Other examples of this include Chevrolet's classic theme, "See the USA in your Chevrolet." It's a great hook that on the surface is very functional, but underneath it promised freedom at a time when America was becoming the most mobile society in the world.
GE's, "We bring good things to life" is another good example of how a theme can become something bigger than just an line at the end of an ad.
Anyone who tells you that theme lines don't matter, has never written a great one.