Get the source code by inspecting or curl.
curl https://xxxxxxxxxx.ctf.hacker101.com/
.
.
<style>
body {
background-image: url("background.png");
}
</style>
.
.
The presence of bg image for a page with white bg seems suspicious. Go to the background.png
using browser or curl, and there's the flag.
curl https://xxxxxxxxxx.ctf.hacker101.com/background.png