My pet peeve is when you're driving and waiting for someone to cross the street but they walk about as slow as a turtle. Certain exceptions apply of course, but if you're physically able and you know a car's waiting to pass, at least put a little pep in your step instead of staring down at your...
Light theme just because that was the default when I first joined, but I can do either without a problem. I tend to switch between light and dark theme depending on the font colors people use. xD
And I'm really glad it's just a simple push of a single button to change themes... not any of that...