Some Forever 21's have a girls department. We've had good luck there... lots of different styles and looks available. My DD is 15 and never been into the glittery stuff either but I'll be honest, I didn't even think that was the style right now. Or maybe it's not the style in our area. Even our Old Navy doesn't have much in the way of glitter.
You might try a nice good-will too. They pretty much weed out all the glitter and logos. You can get some high quality, simple design clothing there for cheap.
Is there an H&M and/or Zara where you live? They both have a pretty good range of styles. H&M is less expensive than Zara. The sizes at Zara run really small and are junior-ish so she may even be able to get some things in an XXS or XS in the ladies sections and not just shop in the kids sections.
I feel for your daughter. 12 is a tough age if you don't run to the "girly" and don't fit womens clothes yet. I was the same at that age, and skinny like a bean pole and remember struggling to find clothes that fit and that I liked.
Good luck, I hope you find some stuff to make her feel really special
Dsd likes aeropostale-I believe they have a girls department some places, though our mall only has the juniors sizes. FYI she may fit some of the juniors shirts in a size 00 or extra small, which is equal to a 10/12 kids. Dsd is also very tall and skinny (though not so much as your dsd, she is 11 and wears a 12 or 14 slim in pants), but she likes the skinny jeans from Target and says they are the best fit, might be worth a try if she hasn't tried them on yet! I was going to recommend H&M as well, or Gap/Old Navy for more classic stuff in kids sizes.
American Eagle has a kids line called 77kids which might be worth a look. Hollister and Abercrombie kids tend to run small and skinny, so if you can stomach those brands, you might look there. I see basically 10 y/o's shopping in Aeropostale, so I'm guessing your dd could fit there.
One of my DD's is in high school and still wears children's sizes.
She likes sports stores -- especially REI. Most of her pants are miniatures of clothes made for women to hiking in. I've found that its worth the money to get something well made that she really likes. She prefers to have a few nice things rather than tons of clothes.