I was sure to have explained it right, but I'll give it one more shot ^^
The goal is just to try out something new. I am not an mmo-c developer though. I have no clue if they are currently working on an update for their website and if dark mode is amongst the list of features or part of any redesign.
I've talked about firefox as it is the tool that I have used and can attest that it works fine on it (not a big chrome fan), but the methods and tools that I have used in the code are fully compatible with chrome and other modern browsers. Sorry if it threw you off when I talked about firefox. I was just trying to encourage anyone to give the style I've made a try and if it works on firefox, then you can usually expect it to work on chrome, that was my way of thought.
Another thing that you have pointed to is that you couldn't clearly see any changes made and you're right, we don't all have the same monitor sizes and as such it's easier to provide a before-after shot rather than what I did so here it goes :
BEFORE
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AFTER
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As you can see the width of the container of the page didn't change, though a lot of borders have been removed, giving more room to the content