These macros exploited CSS injection bug in GitHub. Use any search engine to know about it :) Or see this related video - https://www.youtube.com/watch?v=cC9J-0bvcd8.
$$\ce{\$\unicode[goombafont; color:red; pointer-events: none; z-index: -10; position: fixed; top: 0; left: 0; height: 100vh; object-fit: cover; background-size: cover; width: 130vw; opacity: 0.5; background: url('https://github.com/algomaster99/algomaster99/assets/35191225/8055af3d-c72f-4850-a284-2053d7817439');]{x0000}\$}$$algomaster99 - Overview