Find out more here: http://ryanfitton.co.uk/blog/css3-flat-ui-scrollbars-in-google-chrome/

gmail-scrollbars.css

/* Gmail style scrollbar */
::-webkit-scrollbar {
    width: 12px
}
::-webkit-scrollbar-thumb {
    border-width: 1px 1px 1px 2px
}
::-webkit-scrollbar-track {
    border-width: 0
}
::-webkit-scrollbar {
    height: 16px;
    overflow: visible;
    width: 16px;
}
::-webkit-scrollbar-button {
    height: 0;
    width: 0;
}
::-webkit-scrollbar-track {
    background-clip: padding-box;
    border: solid transparent;
    border-width: 0 0 0 4px;
}
::-webkit-scrollbar-track:horizontal {
    border-width: 4px 0 0
}
::-webkit-scrollbar-track:hover {
    background-color: rgba(220,172,0,.05);
    box-shadow: inset 1px 0 0 rgba(220,172,0,.1);
}
::-webkit-scrollbar-track:horizontal:hover {
    box-shadow: inset 0 1px 0 rgba(220,172,0,.1)
}
::-webkit-scrollbar-track:active {
    background-color: rgba(220,172,0,.05);
    box-shadow: inset 1px 0 0 rgba(220,172,0,.14),inset -1px 0 0 rgba(220,172,0,.07);
}
::-webkit-scrollbar-track:horizontal:active {
    box-shadow: inset 0 1px 0 rgba(220,172,0,.14),inset 0 -1px 0 rgba(220,172,0,.07)
}
::-webkit-scrollbar-thumb {
    background-color: rgba(220,172,0,.2);
    background-clip: padding-box;
    border: solid transparent;
    border-width: 1px 1px 1px 6px;
    min-height: 28px;
    padding: 100px 0 0;
    box-shadow: inset 1px 1px 0 rgba(220,172,0,.1),inset 0 -1px 0 rgba(220,172,0,.07);
}
::-webkit-scrollbar-thumb:horizontal {
    border-width: 6px 1px 1px;
    padding: 0 0 0 100px;
    box-shadow: inset 1px 1px 0 rgba(220,172,0,.1),inset -1px 0 0 rgba(220,172,0,.07);
}
::-webkit-scrollbar-thumb:hover {
    background-color: rgba(220,172,0,.4);
    box-shadow: inset 1px 1px 1px rgba(220,172,0,.25);
}
::-webkit-scrollbar-thumb:active {
    background-color: rgba(220,172,0,0.5);
    box-shadow: inset 1px 1px 3px rgba(220,172,0,0.35);
}
::-webkit-scrollbar-track {
    border-width: 0 1px 0 6px
}
::-webkit-scrollbar-track:horizontal {
    border-width: 6px 0 1px
}
::-webkit-scrollbar-track:hover {
    background-color: rgba(220,172,0,.035);
    box-shadow: inset 1px 1px 0 rgba(220,172,0,.14),inset -1px -1px 0 rgba(220,172,0,.07);
}
::-webkit-scrollbar-thumb {
    border-width: 0 1px 0 6px
}
::-webkit-scrollbar-thumb:horizontal {
    border-width: 6px 0 1px
}
::-webkit-scrollbar-corner {
    background: transparent
}
body::-webkit-scrollbar-track-piece {
    background-clip: padding-box;
    background-color: #f5f5f5;
    border: solid #fff;
    border-width: 0 0 0 3px;
    box-shadow: inset 1px 0 0 rgba(220,172,0,.14),inset -1px 0 0 rgba(220,172,0,.07);
}
body::-webkit-scrollbar-track-piece:horizontal {
    border-width: 3px 0 0;
    box-shadow: inset 0 1px 0 rgba(220,172,0,.14),inset 0 -1px 0 rgba(220,172,0,.07);
}
body::-webkit-scrollbar-thumb {
    border-width: 1px 1px 1px 5px
}
body::-webkit-scrollbar-thumb:horizontal {
    border-width: 5px 1px 1px
}
body::-webkit-scrollbar-corner {
    background-clip: padding-box;
    background-color: #f5f5f5;
    border: solid #fff;
    border-width: 3px 0 0 3px;
    box-shadow: inset 1px 1px 0 rgba(220,172,0,.14);
}