That's a trick they teach you in art school. You can use contrasting colors to make something seem brighter or stand out more than it normally would. In this case, they used blue and orange. A very lovely blue I think.
It also might come naturally 😊 I choose the background color from a stack of colored papers and I found it particularly nice with the tangerine on top.
Edit: my server seems all broken since the 0 19.2-3 debacle so I (Loulou) had to make this reply with another account (the reply doesn't show up with Jerboa)