What clothes should I bring on a backpacking trip

Fleece tops are great because they're warm, dry fast, and can be squished down a lot in a backpack. If you take a fleece top and a lightweight, waterproof jacket, you'll be fine in terms of outerwear.

I agree with Mark Mayo that quick-drying t-shirts and pants that tear/zipper off into shorts are perfect for cutting down on travel. However, you might want to consider getting travel/trekking pants that look a little nicer than the kind that zipper off. That way, you can wear them out to dinner, dancing, etc. Companies like The North Face and ExOfficio make decent-looking ones. You can find trouser-type pants that are waterproof/sunproof and functional but don't look like "sport" pants.

I also suggest getting a camping towel. I like to bring my own towel with me when I travel (and if you're backpacking, I presume you'd be staying in hostel-type places that don't provide towels), I also recommend getting a super-absorbent camping towel. They're basically giant micro-fiber cloths, but they're great because they dry really fast and don't take up much space.

And as Mark Mayo said, how long you're willing to go without doing laundry/having clean clothes is a big factor, too. ;)

I agree with an above comment about writing down what you use and making that your kit. The first time I traveled, I took so much that I didn't need. On each new adventure I was able to pair things down more and more. I recently did a year in South America with a 28 liter bag with room to spare.

Everyone seems to be different so it's difficult to determine what will work for you so all I can offer is generic advice:

When buying clothing, make sure it works in several situations. All black hiking boots don't look too bad if checking out a club in town.

Tan or beige pants that convert into shorts work great for a hike or a night out if dressed up with a polo.

I personally don't like the synthetic stuff. I stick with Smart Wool for all undergarments. I've spent weeks in the jungle with just two pair of socks, shirts, and underwear made with the stuff and never smelled a bit.

Avoid jeans if you can. They are heavy and do poorly when wet. They also often require a belt which adds to your kit.

Try to keep it light as possible unless you're planning to rough it for weeks without civilization. You can always purchase shirts and things if you need. As long as you have a few basics, you'll be OK.

Most other answers have covered all the ground that there is to cover, and I agree with the comments. What I really must say though is that you should add a good sports training jacket to your packing list. NOT one that's too heavy or one of the cheaper ones that are just thin rain jackets. I don't want to name brands but quite a few sports apparel brands have jackets with a mesh-lined inner back with an outer waterproof layer. It works brilliantly in protecting you from cold or windy conditions in the temperature range you've given, as well as offering you protection when it rains.

And, not specifically about clothing, but this will help you carry enough clothes easily: roll up your clothes when packing. It helps in packing everything compactly and you will be able to stuff in at least a couple of tshirts, shorts/jeans etc in even small backpacks.