Minor improvements

Thanks a ton for such an awesome lightbox, very well designed and an awesome user interface. Love it!

Here's what this pull request is about:

* Remove unused executable permissions from the `README.md` and `venobox.css` files
* Make the click area of next/prev gallery navigation buttons bigger

After a few minutes using the lightbox, I noticed that the it was closing when I intended to navigate using the navigation buttons. This is caused due to their small size. On mobile this issue would be more apparent.

The size of the navigation arrows has been increased (with no visual change), so the click area of the nav buttons is bigger, thus resulting on a smoother user experience.