What the fake ubuntu update screen is
Ubuntu’s installer and update screens use a very specific dark purple with orange highlights, and that pairing is instantly recognisable to anyone who has installed it. The Ubuntu typeface completes the look where it is available.
Pranking a Linux user is a specific pleasure, because they will usually be more suspicious than most — the reaction is often "wait, I did not start an update" rather than resignation.
Ubuntu update prank screen online
Press F11 to remove the browser toolbars. On a tiling window manager the browser may already be borderless, in which case there is nothing to hide.
This is a page. It does not touch apt, dpkg, or anything else, and closing the tab ends it.
How to run it, and how to stop it
Press F for fullscreen, then F11 to hide the browser’s own toolbars. That second step is the one that matters: an address bar sitting above the screen gives the whole thing away instantly.
To end it: Escape leaves fullscreen, F11 brings the toolbars back, and closing the tab removes it entirely. Nothing is installed, no file is written and no setting is changed, so there is nothing to undo afterwards.
Worth saying plainly — use this on people who will find it funny, and on machines you are allowed to touch. Show them the way out once the joke has landed.