Use the Rest API to feed content into your site or otherwise interact with WordPress for storing content. http://v2.wp-api.org eventually all this will move into WP Core so V2 is a good place to start.
Then set WP_USE_THEMES to false in your wp-config.php. That will allow you to use WP as your CMS but not load the theme visuals.
From there it’s just a lot of JavaScript calls so something like jQuery or Angular would help.