If you’ve enabled this, no content will be public:
The only way of exposing some Topics, is to make specific Categories private, and de-select this option.
Note by doing so, this will expose some basic user account details like username and names, publicly (but not really sensitive information of course) via API (but not the front end). - similar to the data here: https://meta.discourse.org/latest.json
You might want to disable this option:
For customising the front end take a look through theme-component
However, you might also want to take a look at the: Landing Pages Plugin 
- plugin - Discourse Meta