Highlighter pens can be a useful part of a viva prep toolkit.
You can use one colour to show key references and another to make essential information stand out. You could add a colourful edge to specific pages or mark the start of chapters. It may not be common, but if it meets your needs you could even keep track of typos and future edits with a careful colour selection.
Highlighters are a simple way to show something important but also a clever reminder that you have work in your thesis that matters. You draw attention to what needs to be seen or shared.
As well as edits, references and information, consider using highlighters for the highlights: what do you need to be able to see at a glance to share with your examiners?