Add dark mode

[DennySORA]

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

[Yukaii]

HackMD hero @titangene just ported his dark theme to VSCode!

https://github.com/titangene/vscode-hackmd-dark-theme