Customize HTML pages on server

This is written in the documentation:
“o customize the Rocket.Chat UI you can either modify the rocketchat-theme or rocketchat-ui packages directly, but if you’re keeping in sync with active development it would be easier to avoid conflicts by creating your own theme package.”
I did so and added some test strings in html pages, but i can not see any changes on frontpage.

How can i customize HTML pages on server?