Change color and diff indicating

It's a little harder to see the line number when using dark themes.
So I changed the way of indicating the diff(modified & added) that shows a thin line next to the line number.
I think it's much easier to see the diff in both light and dark themes.
And I also changed the default colors.(This may not be necessary :)
You can check the result:
before:
![before](https://cloud.githubusercontent.com/assets/788863/3787525/10ab3b66-1a20-11e4-882f-419edb34332f.png)
after:
![after](https://cloud.githubusercontent.com/assets/788863/3787526/13d37f42-1a20-11e4-8b5b-4d7f068e8f45.png)