Looking at this:

https://github.com/n2cms/n2cms/blob/master/src/Mvc/Dinamico/Dinamico/Themes/Default/Views/StartPage/Index.cshtml#L6

When trying to create my own Razor templates, @Content doesn't seem to exists, ie- it's underlined red.

Am I missing a using or something?

Sounds like you need some n2 config stuff:

https://github.com/n2cms/n2cms/blob/master/src/Mvc/Dinamico/Dinamico/Themes/Web.config