Logic to make sure scrollbar processing only happens in the editor space...

.... Also fixed issue with scrollbar regex.

https://github.com/adobe/brackets/issues/8482
There's a subtle issue here. If `options.theme` is undefined, then `theme.dark` will be `undefined` rather than `false`. Changing the line in the Theme constructor like so will fix it:

```javascript
this.dark        = options.theme !== undefined && options.theme.dark === true;
```