To access legacy, you should check out the legacy thread in the Announcements section:
viewtopic.php?f=39&t=40052As far as the darkness and such, no, there's no basic way of changing that. For the new version, you can go into the options and turn lighting off or on, but that's all. Modded clients may be able to do more. As such, yes there are mods, and what you want depends on what version of the game you're playing.
Mods for the new version can be found in The Wizards' Tower. For legacy, there are a couple of clients from then that have had the new server information added in, but I don't keep links handy. If you do some searches, there are some graphics and sprite mods in the archives for legacy clients that may still work. I haven't tested any personally.
For future reference, please refrain from double posting. This is a small community. We're generally helpful, but it can take a few hours to get a reply posted.
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.