This is a change that was rolled out earlier this week. To restore the original color scheme, select File > Options and select Colorful from the Office Theme dropdown, then click OK.
You will also notice that the Quick Access Toolbar now has a fixed Save icon on the left, that the ribbon is displayed in a rounded rectangle, and that the scroll bars look different.
This all has to do with making Excel conform to the Windows 11 interface.
My guess is that the other Office apps will follow suit sooner or later.