There has only been one UI change since Snow Leopard that I would consider to be an improvement, and that is the option to enable dark mode. Big Sur reminds me of Duplo Legos.
Finder's background is white by default, but you can change it to whatever color you want. TextEdit and iWork use white backgrounds because they're supposed to be printable. iWeb uses mostly dark backgrounds. Most other apps are mostly a neutral gray.
Dark mode is a bandaid to cover up the fact that modern macOS has too little contrast to begin with. You can make Mojave either too light or too dark, but both options suck.