If you mean you want to change some of the styles then you'll find that in the \theme\klass\scss folder.
Klass theme is a child theme of Boost, so it's basically adding its styles on top of boost (so check out \theme\boost\scss)
What I suggest you do is take the \theme\klass\scss\preset\theme.scss file and copy it - change theme.scss to tarun.scss. Change the styles to suit your needs. See https://docs.moodle.org/dev/SCSS for more details.
After you've made your file, login to Moodle and go to Site administration -> appearance -> Klass you should find your file appears in the Theme preset drop-down. (You might need to purge caches first).