Here's some zip files of the code I wrote in this section:
Dark Mode: https://www.dropbox.com/s/5cvz93eiw29ckmj/DarkModeCode.zip?dl=0
SF Symbols: https://www.dropbox.com/s/m8x22exdvoxtvxs/Symbol%20Fun.zip?dl=0
Swift Package Manager: https://www.dropbox.com/s/s7u2sqwj2wrflb3/Package%20Fun.zip?dl=0
Sign In with Apple: https://www.dropbox.com/s/alf9n33jyzbx32e/SignInFun.zip?dl=0
If you get stuck, it might help comparing your code to mine.