In a piece originally written on GEnie and archived on the excellent Lurker's Guide website, J. Michael Straczynski describes the station as being;
"...patterned physically after the work of such scientists as Gerard
with the central core of the station containing a
"hollow-world look, with fields and hydroponic gardens along the
360-degree circular section (which is about a half-mile, or a mile
across)...This area is known as the Garden."
If you study other O'Neill Cyclinders, you can see that they have alternating glass and land sections to allow light but the B5 gardens seem to be lit using a "sun line", either chanelling light from the nearby Epsilon Eridani star using mirrors or simply creating artificial light using the station's powerful fusion reactors. The lack of an obvious channel between the exterior of the station and the interior would strongly suggest that it's the latter.
You can see the "sun lines" in the pictures and video below;