Thanks for all the replies!
@Timoshwarzer: I tried scaling the game to match the WorldEnvironment fog, but that wasn't working too. I'm not too sure how WorldEvironment fog even work; the game is on my GitHub account, if you want to check it out: https://github.com/henriquelalves/simple_dungeon_demo
@MrMonk: Thanks, I'll check it out! I'm still unsure on where I should put the script, thought, haha.
@Zylann: Well, I'm working on normal "Godot units", which in Blender should be a meter each. But I tested changing the scale of the game and the distance of the fog on the WorldEnvironment node, and now I'm just confused on how does it work. If you want to check it out, the game is on my GitHub: https://github.com/henriquelalves/simple_dungeon_demo
@CraftyManiac: I didn't thought on using viewports, actually; apparently I can add an Environment property to the viewport itself, so maybe I can use this to create the fog Effect that I want. A particle emitter probably wouldn't work, because the shader would be on each particle material (I guess, I'm not even sure on this one haha).