Match the dark theme with the one used by GitHub

[faheel]

The dark theme used in this extension is different from the one used by GitHub. It would be great if they both matched!

Also, when the dark mode is enabled, there is a white border around the editor which doesn't look good.

[faheel]

The defineTheme method can be used for this.

[hediet]

That would be a nice addition indeed! I won't find time for this in the next couple of months though. I welcome PRs!

[faheel]

I'll dig into this more over the weekend. Hopefully I'll be able to implement it.