To configure a Theme for HTML5 Gateway page read here
By configuring a Windows client theme, you can provide a Parallels Windows Client with custom branding to your customers and/or users. To manage themes:
In the Parallels RAS Console, navigate to Farm / Site / Themes .
Note: The Themes view in the right pane displays the available themes. The list contains at least one default theme. This theme cannot be removed but you can customize it as needed. In addition to the default theme, you can create your own themes
To create a new or modify an existing theme:
Click Tasks > New Theme (or click the [+] icon) to create a new theme.
Double-click an existing theme (or right-click it and choose Properties).
The Theme Properties dialog opens. Use the dialog to create a new or modify an existing theme.
On the Branding pane, specify the following:
- Company name: Used to create the Start menu hierarchy: Start \ Company Name \ App Name.
- Application name: Displayed in the app caption and the Start menu entry name.
- Connection banner: Displayed when a connection is being established.
- Application icon: The application icon used for the Start menu and by the main app window.
To override the default post-logon message, select the Override post-logon message option and enter a message.