The user can add one if they want to, but it is best to provide a default favicon because it makes it easier to recognise a website in many different places. It can aid with locating the website amongst a list, and also the particular tab in a browser.
With a custom stylesheet, users will be able to take the default theme and re-model it how they like; but I will keep open the option of different themes entirely (that is, being able to specify completely different HTML), because it is very useful for doing translations or integrating the forum with an existing site design (a common requirement).