Alright, I don't know if this is possible but I'm wondering if there's custom CSS to change the background of the site (like, the main white part) to a light grey color instead? I don't want the colors completely inverted, because that tends to hurt my eyes, but I'm switching between Subeta and another game right now, which I'm training in a dark area so the white is completely blinding.

I've tried looking for grayscale Chrome extensions but they still use white (or too light of a color, at least) for the main part of the page, which is where the problem is in the first place :/ Any ideas, either for CSS or an extension of some sort? :x

I don't know if that's possible, but did you already tried the Custom CSS part? I have not enough knowledge about it, so I'm not totaly helpful. Also, there's an extension called greasemonkey, but I also don't know how to deal with CSS onsite x_x

This is the code I use to change the site colour, {background-color:}

That's a soft cream colour that just barely shows. Its takes down the white brilliance just a bit without making the areas that stay white stand out too much (like the backgrounds of items, HA's etc.). You can change it to which ever colour suits you best, just look up the hex code, or change the to a named colour like Silver, Gray or DarkGray (which, strangely enough is a lighter colour than Gray).

I'm not good with coding, so I wasn't able to change anything with it

Thanks! You're a lifesaver :D