What you *never* do in Moodle is modify core files. That makes it a real pain to upgrade and maintain.
You have some choices here...
* If you are using the "standard" Boost theme (you are by default), you can add custom CSS in the theme's settings page - see Boost_theme
* Change to one of the may custom themes available - see https://moodle.org/plugins/browse.php?list=category&id=3
* Write your own theme - https://docs.moodle.org/dev/Themes
You have some choices here...
* If you are using the "standard" Boost theme (you are by default), you can add custom CSS in the theme's settings page - see Boost_theme
* Change to one of the may custom themes available - see https://moodle.org/plugins/browse.php?list=category&id=3
* Write your own theme - https://docs.moodle.org/dev/Themes