Well....
1) Do you have shared finances where a purchase like this would come out of the money pool? If yes, is she asking you to pick them up because it's more convenient for you to get them, maybe you drive past that particular store on your way home from work?
2) Do you have separate finances and she wants you to pick them up because it's convenient for you and you guys can balance it out after?
3) Do you have separate finances and she just wants you to pay for them cause she's broke.
4) Did you buy your game using allotted budget money, or were you willing to go outside of your budget for something you liked?
I love Christmas lights...not Chevy Chase kinda lights, but strings a twinkly white lights make me happy. In fact I just bought 2 new long strings this weekend cause they were on sale. If I'd asked my hubby to pick them up for me, he would have, but he would have also asked me where we they fit in our budget (we just restarted the MMM thing so we're being a bit militant which each other). But I bought them using my monthly personal money and the store was on my way home. Aaannnd I put them up.
The comments in this thread are interesting because it seems most people are saying "Buy Them", keep her happy. But really that seems to go against everything I'm learning on this site. The "buy things to keep me happy" mentality is what gets people into financial hot water. Sure maybe all in all, she just wanted 50$ worth of lights...not really a big deal in the grand scheme of things, but the point is to question your purchases, do you need it or do you just want it. If you need it, buy it, but if it's just a want, figure out where that purchase fits in your financial story and decide if you have the funds and/or flexibility to spend that cash.
If it's not so much the money aspect, and she has some wiggle room in her spending this month to buy them, and it's more about the fact that you don't want to put them up, then in that case I say, Keep her happy. Put up the lights. That way she won't feel the rage building inside her over the holidays every time she looks outside and doesn't see twinkly lights while you are hunched over your game console playing Call of Duty.