Add color-scheme for dark mode

Created by: robinheidrich

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Bootstrap should make use of color-scheme to display browser elements (scrollbar, select, etc.) in dark mode as well.

Motivation and context

Here's a preview of what scrollbars currently look like in the Bootstrap documentation, and what they would look like if color scheme were used.