I turned it off, Why still transparent??

[sameerasw]

TL;DR How to fix

If you were using the Transparent Zen mod, it's because you didn't read the clear instructions... Turn off the options in the mod before uninstalling and then re-start the browser to take the disabling transparency to take effect.

If you didn't use transparency before, it's because you have the flag browser.tabs.allow_transparent_browser -> true in about:config .... switch that to false and re-start the browser. Now the web content will not be allowed to be transparent.

If you want other websites to be transparent but not this one, Use the userContent.css or a 3rd party addon to add a simple background body, html{background-color: light-dark(#fff, #222)} to the hody of the webpage manually.
I am working on a workaround from my end which might take a bit of time... :)

Why it happens?

It's because the mod is there to mainly enable this flag browser.tabs.allow_transparent_browser to remove the default colored solid back-plate from the website view of the browser. But usually the page is intact since it got a background color for the body, html etc. But, some websites for some reason skipped this like Google Classroom which automatically achieves transparency since the browser isn't providing a back-plate anymore since we disable it.

So simply it becomes transparent without even an add-on, Turn off the flag to add that website back-plate back.

Why that flag is enabled?

In the Transparent Zen, well... it's said in the name... to allow the websites to be transparent with the Transparent Zen Mod + Zen Internet Addon. If you didn't use it, maybe another mod enabled it for some reason or you might have accidentally done it... These changes in mods aren't automatically reverted upon uninstallation so that's why they persist even after uninstalling.