[Typo3] Collapse menu tree except active subdirectory

[Scott Ott]

I'm creating a website that has several dozen websites in subdirectories 
within it. They'll share a single URL (although I would prefer 
subdomains to subdirectories, that can get pricey.)

I would like the menu tree in the left margin to expand for only a 
single subsite at a time--the rest of the menu displaying only the top 
level. The site is three layers down in the tree. Like this...

Main
 > Subsites
 >> State
 >>> Subsite Home
 >>>>About Us
 >>>>Contact
 >>>>Directions
 >>>>Etc.

When a particular Subsite Home is active, I want the other states and 
subsites to be invisible, so it the menu would show ...
Main
 >Subsites
 >> State X
 >>>Subsite Home Y
 >>>>About Us Y
 >>>>Contact Y
 >>>>Directions Y
 >>>>Etc. Y

Is this an issue within the CSS of the template that I'm using, or can I 
configure it in Typo3 BE admin.

Thank You,
Scott Ott