nebulous

depends on the game, really

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there's https://sfx.thelazy.net/games/ but it's a list of edited game files to add shaders and such

else you need to find shaders mods for that individual game (i.e. shaders for minecraft, garry's mod...)

reshade?

ReShade and SweetFx do wonders, but getting the settings right is a pain in the ass.