It gets warmer towards the downtown (center of the city) but once you're anywhere near the coastline to the west or even near the Bay, it gets foggy, misty, gloomy and cold. There's a stark contrast with just a difference of a half mile, so pack in layers. Also consider that when it gets warm, it gets WARM because a lot of the older buildings don't have air conditioning so you can find yourself roasting in some boutique or indie coffee shop.
There's an old quote that goes ""The coldest winter I ever saw was the summer I spent in San Francisco." People attribute it to Mark Twain, but it's not authenticated. What is right about it is that it's FREEZING in San Francisco during the summer. It's the hot air inland mixing with the cold air from the Pacific Ocean, which leads to never ending fog and low temperatures. When you get away from the ocean, it gets warmer but then you find yourself at the Bay which can lead to wonky weather anyways. If you're going to the East, North or South Bay, temperatures are more consistent (typically California summer weather, relatively dry, warm and sunny). It's the city that will drive you crazy.
So, yes pack in layers. I would suggest a sleeveless top, cardigan and lightweight jacket for shopping. Think that if you shed layers, you have to carry it around. Jeans or leggings would be good, short shorts or miniskirts at this time of year would be a bad idea. Day dress or long skirts around knee length might be okay if you're a trooper and don't mind drafty knees, but then again it might be a good idea if it really does warm up this weekend.
As for shoes, close toe flats or boots would probably serve you better. I went to SF in late June and wore gladiator sandals and my toes nearly froze off at times. It's hilarious because I lived in SF for 4 years, so I should have known better!
I think black for nighttime is fine for summer. Wear a shawl, jacket or thicker cardigan, it gets cold EVERYWHERE at night. Pumps would probably be better to keep your feet sane compared to strappy sandals.
No matter where you go, unless you do valet you'll be hard pressed to find parking right next to where you plan on going (shopping, dining, whatever). So be prepared to walk a couple blocks from your car to the destination, so no "these are my car -> sit down dinner shoes!". Comfort is key in San Francisco. Not even considering the hills, sometimes there are no sidewalks, there are a few construction sites downtown and the road quality is kind of shoddy.
Have fun!