Support default dark theme in sources panel in Chrome v50

[micjamking]

In Chrome Version 50.0.2642.0 canary (64-bit); there is a default 'dark' theme available, skinning all panels in DevTools with a Dark theme.

DevTools Author works out of the box with the new dark themes, only targeting code-view and console panels within DevTools and targeting syntax highlighting within that.

However, DevTools Author also skins the sidebar panel using a slightly lighter 'gray' background color, which directly impacts the legibility within the sources view

It looks like the new DevTools themes use a body class of .-theme-with-dark-background to add styles, so adjusting the support for v50 should be fairly straightforward.

Reference Issues: #31

[micjamking]

If possible, it would be nice to also skin the Author Settings panel to use a dark background when the 'dark' parent theme is applied.