The arguments and reasoning are the real substance of the text, so they're pretty important.
Let's highlight Restall's main claims as well as how he supports them:
Well... that's quite a lot of yellow. I sometimes fall into this trap, too. While it might be tempting to highlight everything that seems moderately important, after a certain point, the highlighter loses its purpose of making the actually important parts stand out. Instead, it just makes any non-highlighted portions get lost in the field of yellow while also drowning out the author's main ideas. When we go back to review this text, it'll be difficult to see what Restall is arguing without rustling through practically the entire work again as if it was the first read-through.
Let's try just highlighting the author's main claims instead: