PHB, page 175:
Passive Checks
###Passive Checks ...
Here's how to determine a character's total for a passive check:
10 + all modifiers that normally apply to the check.
If the character has advantage on the check, add 5. For disadvantage, subtract 5.
Since dim light gives you disadvantage on Wisdom (Perception) checks, a character without darkvision would subtract 5 from their normal passive wisdom perception score.
As for darkvision:
A creature can see in dim light within the radius as if it were bright light.
So they wouldn't have disadvantage and would use their normal passive score.