In the resources/deeplink directory you will find two .html templates which you can modify/ style as you like, as long as you leave the template keys intact.
The indexpage constant is not for styling, but to indicate you want to use an alternative index page. That is especially useful when using some kind of single sign on.
Don't know if this is want you want but you can set the indexPage property. (Quote documentation)
Set the IndexPage constant (new in 1.3) By default, the IndexPage constant can remain 'index.html', however in special cases, such as when you want to load a specific theme, or bypass a certain single sign on page, you can modify this constant to for example 'index3.html' or 'index-mytheme.html'