I hate when people generalize or oversimplify things.
That said, here are a bunch of TV Shows' themes summed up in one sentence:
Buffy the Vampire Slayer--Life is hard.
Angel--Don't stop fighting.
The Shield--The wages of sin is death.
Seinfeld--There's a pretty steep price for doing nothing.
Lost--Anyone can be redeemed.
South Park--Seriously, just chill out.
The X-Files--The Truth is the meeting ground of skeptic and believer.
The Sopranos--Then it comes to be that the soothing light at the end of your tunnel was just a freight train coming your way.
Frasier--Smart people have problems.
King of the Hill--West side is the best side.
Any other ideas? Corrections? Peanut Butter Jelly Time?