Most light fittings have a slight "standoff" at the screw holes, just
about enough to clear 1 mm T&E. If coming up from below, I'd just add a
blob of silicone if plastic, or rubber grommet plus silicone if not.
If wire comes down from above, you don't want it to have any chance of
draining water into the light. You could increase the standoff slightly
using washers or grommets on the screws, then arrange the feed to have a
loop below the inlet hole, then seal as described.
Sometimes worth having a drain hole at the bottom of the light fitting
anyway.