Add dark mode

**Is your feature request related to a problem? Please describe.**
Use VScode half is dark and HackMD display is white,i feel weird.

**Describe the solution you'd like**
want to add dark mode.

**Describe alternatives you've considered**
nope

**Additional context**
none

The issue names no files, tests, or entry points. Start by locating how the VS Code extension renders HackMD content and how it handles VS Code theme changes; done should mean the display offers or follows a dark mode consistently.

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