How to customize the new Marina theme Portal Home page

We are using the Multiple Product feature, and we currently have 3 different products (brands) each with their own Portal URL, email address and ticket form. We will soon be supporting another product, and we are now having customers that will be using more than one product. This is causing confusion among our customers, and we would like to offer only one landing page (our native Portal Home) to reach us and allow them, from this page, to select (using a button or any other way) the product they need help and be redirected to the appropriate product (brand) Portal URL.

Is it possible to customize our native Portal Home page (we are using the Marina theme) to perform this redirection to another portal URL and if it is possible, how can we achieve this?

With the old Mint theme, we had documentation to help us out customizing our Portals, but for the Marina theme, we cannot seem to be able to find any customization information.

Thank you!