I can see three representations in the grid.
One of them is a ten letter word
The second is a mathematical expression 'in words'
The third is a mathematical expression in which the letters are interpreted as numbers. Strictly speaking there is an ambiguity which requires the use of parentheses to resolve, but I think mathematicians would agree on which interpretation to accept, on the basis that the alternative could be represented in another way.
So I appear to have 23 cells to highlight. Two of them are use twice.
I certainly don't see why the unclued answer 'contains the theme', although I would be highlighting its final letter.
And I cannot make a great deal of sense of either the corrections or initial letters of clues 'in normal order'. In a crossword, initial letters of two lights are given only one number. The clues are then listed in two groups - first the across clues and then the down clues. This does not give me anything very useful though there are bits and pieces which I expected to see.
Having spent a great deal of time on the jigsaw, much of which wasted since I had the long words wrongly placed, I am now feeling a bit cheesed off at these final ambiguities.