Almost muddled my way through to (nearly) the end. Spent too long investigating the puzzle title!
My final brick wall is the two six-letter unclued entries. What exactly does "specified by the letters from their crossing answers" mean?
I can find an entry (which may be thematically relevant) for 22 across but I cannot find any connection with the three letters originally there.
The other unclued light has foxed me totally.
I'll make no comment about the high lighting!