Expand use of Primary Color to include h2 and border under hero image

My theme (Esteem) uses Wordpress Customizer to select a Primary Color. Right now the Primary Color is applied to links and buttons. I would like to expand its use to the border below the hero image and the H2 tag. How can I do this? Yes, I am using a Child Theme, and expect that these changes will go in the functions.php file of the child theme.