[pickers] Improve dark mode colors

[joserodolfofreitas]

Summary 💡

Extracted from #7440

Open https://next.mui.com/x/react-date-pickers/date-picker/#basic-usage, the background of the days should be transparent, replacing the custom style we apply and the browser default, e.g. background-color: buttonface; on Chrome.

Same for the date range pickers, the design is broken. The range marker should be less opaque.

how it should be:

Motivation

Polish the component's color range.