Working as intended. Sometimes the search won't show up if I try adding to playlist from hamburger menu on the front page of YouTube, or via search results, but it always works from the video page directly. Clean and compact too, so it's super unobtrusive. Exactly what I wanted.

I̶ ̶t̶h̶i̶n̶k̶ ̶t̶h̶e̶r̶e̶'̶s̶ ̶a̶ ̶b̶u̶g̶ ̶w̶i̶t̶h̶ ̶t̶h̶e̶ ̶s̶e̶a̶r̶c̶h̶ ̶b̶a̶r̶ ̶c̶o̶l̶o̶r̶,̶ ̶i̶t̶ ̶s̶t̶a̶y̶s̶ ̶b̶l̶a̶c̶k̶ ̶e̶v̶e̶n̶ ̶i̶n̶ ̶l̶i̶g̶h̶t̶ ̶t̶h̶e̶m̶e̶.̶ → Fixed now. Thanks!

Still, cool! Just what I was looking for.