How can the css be edited?
You can make a custom extension, or upload custom less to the admin dashboard
What is the less to upload to the admin dashboard to remove the welcome hero and discussion hero? Or better yet to reduce their height?
I believe Clark shared this here: https://discuss.flarum.org/d/24356-how-can-the-landing-welcome-message-box-be-set-to-off/2
For the discussion hero, it's pretty much the same. Use inspect element to figure out which elements exactly you want to hide and use their css classes.