I AM TRYING TO ADD A NEW bootstrap.css file to the site. I created a theme but not sure how to add it. I thought that I would be able to select it from the “system settings page”. See other screenshot. How do I do this?

I have the following files I’d like to add to change the color theme:
_variables.scss
bootstrap.css
bootstrap.min.css
custom.sass

PLEASE HELP.
Thank you,
Brent

Screenshots:
https://www.screencast.com/t/WMSqOGAEO
https://www.screencast.com/t/TNCC2LQc8Bdn