Can this form setup be modified? for example, can you modify css and styling?
Did you possibly mean "forum" instead of "form"? If yes, then yes.
The forum script has an (admittedly outdated) default theme, that can get replaced by an own theme (possibly based on the default theme) with all the wanted changes to the HTML-structure and the styling with CSS. The theming feature uses the Smarty-engine. So the functionallity is more or less well documented.
It is also possible to apply the wanted changes to the default theme but in this case it is accidently possible to overwrite those changes with an update to a newer forum version. Because of that it is recommended to copy the theme to a new one and to activate the new theme only for one self during the development and testing phase.
PS: What has this to do with requiring a login?
Trenne niemals Müll, denn er hat nur eine Silbe!