The light levels in UOX3 can be controlled either globally, or locally on a region-per-region basis. The global lightlevel commands are as follows:
'DARKLIGHT # - The darkest light-level that is used in the day/night cycle.
'BRIGHTLIGHT # - The brightest light-level that is used in the day/night cycle.
'DUNGEONLIGHT # - The default light-level in dungeon-areas (The right-most area on the map, where all the dungeons are located).
However, you can override these global light-levels on a regional basis. For instance, if you have a specific region on your map (Regions are defined in UOX3/DFNDATA/REGIONS/regions.dfn) that you want to be generally darker than the rest of the world, you must first define a new weather-zone in UOX3/DFNDATA/WEATHER/weatherab.dfn.
These weather-zone are mainly used for controlling the weather (which is covered by the next section in this guide, 3.1 - Climate), but we'll create a zone here which covers light-levels only. Here's our example zone:
The important part of the first line is '6', which is the unique identifier-number for our new weather-zone. Simple really. Just take the number of the (previously) last region in the file, and add one.
Next up is LIGHTMIN=20 and LIGHTMAX=10. These indicate the minimum and maximum lightlevels of the weather-zone. LIGHTMIN is the setting for the darkest light-level in the zone, while LIGHTMAX is the brightest light-level in the zone. So by setting those values to 20 and 10 respectively, we make our weather-zone a pretty dark zone, which never gets brighter than '10' and never darker than '20' - which is the darkest possible light-level.
Now, to assign this weather-zone to a region in our world, we must open regions.dfn, locate the region we wish to modify, and add a line in it's section like this: ABWEATH=6
NAME=the town of Serpent's Hold
ABWEATH=6 <-- There's our weather-zone definition!
Now this region, in this case the town of Serpent's Hold, will have light-levels independent from that of the rest of our world!