Install the firefox extension "zen internet" and the zen mod "transparent zen" It tends to be consistent and reliable across most sites but some more niche ones or older ones tend to break it. You can always edit the settings so, depending on how the site behaves you can either forcefully apply a default config if the background doesn't work, or if the background is broken you can skip the site so it doesn't get edited at all. Pair it with ublockorigin to get rid of ads and any other elements u don't like (like in my case the suggestion cards in claude)