I did not use Rehub theme, but it should be adding a Rehub menu on the Wordpress dashboard (after you log in). From there you can make modifications to the theme and set preferences. (Google Rehub documentation for help)
Most theme like this will also offer "custom css" or "custom codes" from the dashboard theme's settings.
If those are not enough for what you need, then you can locate the files to modify them yourself (Html, Css, Php, etc.)
For this, go to the Wordpress dashboard, and from the left menu go to Appearance -> Editor and there will be a list of the files used by the theme on the right and you can modify them there or copy them to notepad (or any editor). Copy back on the dashboard and save when you are done.
If you prefer using FTP to fetch the files, then the files will be in ../root/wp-content/themes/"theme's name"/css (or anything you need) on your host's server.
Good luck