You would create /opt/bitnami/apps/moodle/htdocs/theme/boost/templates/core_course/activity_navigation.mustache. You may need to create the core_course directory first.
Are you sure you have Boost theme selected and not a child theme? The CSS should work unless there are some errors or conflicts with some other custom CSS.
Are you sure you have Boost theme selected and not a child theme? The CSS should work unless there are some errors or conflicts with some other custom CSS.