https://wiki.hard-light.net/index.php/Lighting_Profiles.tbl has a full description. By making such a table and putting it in your root data/tables directory it will overwrite/modify any downstream mod.
If anyone ends up feeling they really,really need that fine grained control of making different lights brighter, dimming all the suns and boosting the dynamic lights or whatnot, an important piece of the planned feature set is going in tonight.
What Asteroth suggests to do is common wisdom, I've seen it suggested for various things over time by many people since I joined the community, but it is not actually correct. Sticking things in that location is fine for adding to mods or overiding the base game, but both reading the code and trying some clear test cases has shown me that if you have the same file, or the same value, in both a mod's data/tables and fs2/data/tables, the mod wins. So that's no good for user customization if mods start adopting tables.
The feature added tonight is a new flag, -override_data. When present this activates a new step in the load order that will always be after the mods. With this flag active, anything in fs2/_override/data/ will be loaded after the base game and all mods, so people can put tweak tables in there, and share those tables, if they feel that need. This opens up a lot more fine grained tuning over things like light sizes, or lights from different types of weapons, that the specular flags did not provide at all.
I hope to eventually add some more features to lighting profiles intended to be used with this, so you can adjust relative to the mod's choices rather than just hard set your own values, but for now you have this at least. Mods can set their lighting identity more concretely, without the need for launcher flags, and tweakers can still take that control back if they feel the need.