You can use ambient lighting during the build process before you've put in any lights to get even lighting across the entire map.
Select worldspawn, enter the key ambient and a low value, such as 50 or 100.
Also note that you can have no light entities in your map whatsoever for ambient light to work. Also note that ambient lighting, for the most part, looks awful and should not be used on the actual release of a map.
There are three things that I paricularly hate: kids, animals, and women with attitudes.
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