Here's a way to create a white diagonal line on a black background using pseudo elements and css triangles. It works on all modern browsers, and IE8.

diagonal-lines.sass

// Diagonal Lines
.some-element {
    background: black;
    position: relative;
    &:before,
    &:after {
        border-top: 40px solid transparent;
        border-bottom: 40px solid transparent;
        content: "";
        display: block;
        height: 0;
        position: absolute;
        top: -40px;
        width: 0;
    }
    &:before {
        border-left: 40px solid white;
        left: 1px;
    }
    &:after {
        border-left: 40px solid black;
    }
}