So when I use the

"Change Screen Colour Tone"

option, it also changes the colour tone of the battles. The enemies and backgrounds become super dark; how do I fix this :/?

There should be a way to fiddle with the color tone during the battle.
I know there was battle eventing in regular VX (through troops), I'd simply add another event that changes the tone back to normal.
Sorry that I'm not very specific.
I haven't actually fiddled around with the engine as much as others have.

(Then if you have the problem with it not being dark coming back to the map, create a parallel process on that map that changes the tone.)

I tried that, but it didn't work unfortunately. It's still super dark

PC problem, probably?

Na. It's the same for me. Personally, I don't really care much, only when it gets to the very dark areas.
I'm pretty sure there's a way to separate the tint between the map and the battle. They used this in RPG 2003

I could do it via the monster panel in VX, but it doesn't work in VX Ace. It's a bother when in forests / caves :/

Well, depends. Do you plan on completely separating battle screen tone from map screen tone? If so, all you need to do is change this one line in the Game_Troop class:

to the following line:

And it should work as expected. Now, if you plan on having one or more battles somewhere in the game that require you to keep the same screen tone used in the map, the easiest way to set it up is by using a switch. Instead of the previous example, replace the line above with this one:

Where (number) should be the actual ID of your control switch. So, if you choose to put in 20 as the switch ID, the battle screen tone will only be the same as the map's if switch 20 happens to be on.

Hope this helps!