I wish to know what happened too.
Like I said, search the forums. I googled for "yosemite dark mode stuck", and this MacRumors thread was THE second result on Google's list:
https://forums.macrumors.com/threads/1762059/
Even if you don't have Lights Out installed, there is a terminal command that someone gave in the last post that supposedly fixes the Dark Mode checkbox. It's worth a try.
Not sure what to say about the other two issues. Maybe they're related to the Dark Mode problem, but then again maybe they're not...
So I installed PB4, now Safari, Appstore, Ping etc can't see the internet. DNS issue since I can open a webpage with IP. I check DNS settings and they are all correct. Looks like my DNS isn't running in Safari and Webkit. Chrome works though
I ran into a super-weird problem with a Mavericks upgrade on a friend's MacBook recently that sounds eerily similar to what you are describing. They were all the way back on Leopard (!) and I did a direct upgrade to Mavericks from a thumb drive installer. Everything appeared to be fine except I could not make any of the network-aware apps included in the OS work...Safari, App Store, etc. They would all just spin and spin and spin. I could do DNS lookups from the terminal and ping the same hosts just fine. I put Chrome on a thumb drive and installed it on the machine, and it could pull up web sites just fine! Baffling. (I don't remember if I tried going to a web site via IP address in Safari or not. It might not have occurred to me because the symptoms were unusual...Safari didn't say it couldn't find the server, it just sat there and acted like something was loading but nothing would ever show up. And I could resolve DNS on the command-line.)
I started poking around in network settings, and found that the machine had some DNS servers statically specified in the config for the Wi-Fi adapter. On a whim, I removed those and had the computer instead learn its DNS from their router's DHCP server. Problem was IMMEDIATELY fixed. To this day, I cannot explain it. They were obviously using the machine with their DNS configured like that when it was running Leopard and were not experiencing these kinds of problems, so I don't know why those same DNS servers would suddenly become unusable after the upgrade.
-- Nathan