I’m afraid you will not find any CSS inside OJS. It uses Less that is compiled in the cache folder into usual CSS. But you can download custom CSS if specify a path to it inside child theme. Example of creating such theme is provided through the link above.
in theory… but I do not do this yet! maybe at some time after to have the system more mature, I am just configuring the workflow and try to help the editors to understand the system. If you are lucky making some changes in the theme or made a child theme, you are welcome to share your experience!