Make NavBar Re-appear on Upward Scroll

Enter your suggestions in details:

While reading a docs page, I decided to switch to the dark mode. But the mode button is not in the right section, it's in the navigation and navigation is hidden unless I scroll to the very top of the page, loosing the position I was reading at.

One possible solution would be to move mode switch to the right section (where ToC is). Another is to make navigation reappear whenever user starts scrolling upwards.

Other options to improve mode switching that come to mind are:

1. Add the "system" mode option that follows the light/dark OS setting, like the one is MacOS.
2. Add a hotkey to switch mode.

首先检查文档页面导航和深色模式控件，然后确定项目应采用所提议方案中的哪一种。选定的模式切换改进能够正常工作，且无需强迫读者返回顶部，并由相关的 UI 行为检查覆盖，即视为完成。