Does the Outlands have a Sun & a Moon? I'm assuming it has day & night (the art seems to indicate so anyway)... is it just an indistinct illumination and then darkening, or are there actual celestial bodies (or the image of them anyway) in the sky overhead? If so, over which horizons do they rise and set & is it at a stable rate (12 hours of day & 12 hours of night) or are there seasons where day & night grow longer and shorter through the year? If I missed this info anywhere, please point it out to me!
Similar question applies to Elysium, Arborea, & Ysgard.
The Outlands has no celestial bodies. As with Sigil, the sky lights up until Peak and then gets dark until Anti-Peak. The light comes from the sky in general rather than any specific celestial body.
The only Outer Planes noted to have anything like a sun or a moon are Arcadia (which has the Orb of Day and Night, which stays at "noon" position and goes dark for twelve hours of the day), the Beastlands (whose sun Selara is eternally at "noon" in Krigala, whose moon Noctos is always straight overhead in Karasuthra, and whose sun and moon are both at the horizon in Brux) and some of the layers of the Abyss. And, of course, Carceri with its orbs and Gehenna with its distant volcano layers, and Ysgard with its rivers of earth.
Most outer planes either rely on indistinct light from the sky or from whatever suns and moons are in the sky of the neighboring sun and moon gods. For example, in Elysium near Pelor's realm, Pelor maintains a sun. The Indian god Savitri maintains a sun of his own in his realm, while Tsukuyomi maintains a moon.
Ysgard is lit by the fiery undersides of the earthbergs above the one where you're on. Carceri is lit by other orbs. Gehenna is lit by the volcano-layers in the distant void (as well as whatever molten lava and pyroclastic activity is nearby).
This is all from the original Manual of the Planes and The Player's Primer to the Outlands. And Planes of Conflict, in the case of Gehenna.
I'd also like to point out that because most of the Outer Planes are infinite expanses of land, it's not possible for a celestial body to "set." Unlike in a Prime world, they'd just run into the ground. That would be okay if the sun god has a palace to park his chariot in, but it doesn't work if the sun is a big ball of fire that incinerates the forests and boils the seas. Of course, Carceri, Gehenna, and Ysgard all have plenty of void for celestial bodies to be in, but they're exceptions and they've already been covered. I guess the sun and moon in the Beastlands are always low in the sky, but don't dip below a "horizon" that doesn't really exist, but then again I suppose things don't have to make sense in the Outer Planes.