gistfile1.txt

// Sets container background to black
.thumbnail .thumb_image {
    background-color: black;
}

// Sets the 'mouseout' transition speed
.thumb_image {
  transition: opacity 0.25s ease-out;
}

// Sets the 'mousein' transition attribute and speed
// You can play with opacity to your liking (Scale from 0 - 1)
.thumb_image img:hover {
    transition: opacity 0.25s ease-out;
    opacity: 0.8;
}