Improve caret visibility

Love the theme. After using it for a couple of days though I find it's very hard to see where your cursor is if you're not following carefully - especially if you're trying to jump to a point on a line.

This changes the caret colour to #FFFFF from #F8F8F0. IMHO this looks good and makes the cursor very easy to spot.

![screenshot](http://i.imgur.com/2DtHaX2.png)