Thank you, RueNel!
Yes! I’m trying to copy the custom CSS in Pro > Theme Options > Custom CSS! I’ve done this so many times over the years and never ran into this so I’m perplexed! I had actually already tried copying and pasting into Notepad too, and it still brought the line numbers with it!
Oddly, I also just noticed that only PART of the CSS will paste. I did Ctrl-A to select all (confirming all was selected) then copied and pasted into Notepad, and it only pasted from line 103 to the end. I checked to see if something was askew in the area of that line and even deleted that section of CSS (even though it looked fine) and no matter what I deleted or moved, I was still only getting what appeared to be a random piece of the full CSS! (Which thankfully isn’t too long yet!) 
I also went to two of my other sites and tried copying/pasting the CSS to be sure it wasn’t happening everywhere all of a sudden, and those two worked perfectly.
I’m thinking it might be a corrupt file! I’m open to suggestions!!
Thank you again for your help!! Much appreciated!!