Hi Kara,
You can add this manually to the code by going to Appearance -> Editor. You will need to change two files.
In Theme Functions, find the area near the bottom where all the social media settings are defined. You can use these as models to create a $wp_customize->add_setting
and $wp_customize->add_control
call.
In Theme Footer, you will have to add two pieces as well. You will have to set the $youtube or $pinterest variable first (something like $youtube = get_theme_mod('rokophotolite_youtube_link');
, depending on what you put into the theme functions. Then add a statement that actually generates the icon based on the code you see for the other social media icons.
Hope this helps!