CSS scrollbar style cross browser

Scrollbar CSS styles are an oddity invented by Microsoft developers. They are not part of the W3C standard for CSS and therefore most browsers just ignore them.

Webkit's support for scrollbars is quite sophisticated. This CSS gives a very minimal scrollbar, with a light grey track and a darker thumb:

::-webkit-scrollbar
{
  width: 12px;  /* for vertical scrollbars */
  height: 12px; /* for horizontal scrollbars */
}

::-webkit-scrollbar-track
{
  background: rgba(0, 0, 0, 0.1);
}

::-webkit-scrollbar-thumb
{
  background: rgba(0, 0, 0, 0.5);
}

This answer is a fantastic source of additional information.