How to fix bookmarks level

I have multiple book designs, some of the books have part then chapter. So that I have added the CSS, the part as a 1st level(prince-bookmark-level:1), and the chapter as a 2nd level(prince-bookmark-level:2);

Some book doesn’t have the part so I need to change the chapter as 1st level(prince-bookmark-level:1);.

How to control this in the same CSS.

Please let me know if Prince has any option for cases.

You could create two rules, one which checks for the previous heading using the sibling selector.