How can I increase a scrollbar's width using CSS?

This can be done in WebKit-based browsers (such as Chrome and Safari) with only CSS:

::-webkit-scrollbar {
    width: 2em;
    height: 2em
}
::-webkit-scrollbar-button {
    background: #ccc
}
::-webkit-scrollbar-track-piece {
    background: #888
}
::-webkit-scrollbar-thumb {
    background: #eee
}​

JSFiddle Demo

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References:

• Custom Scrollbars in WebKit | CSS-Tricks
• WebKit scrollbar demo from CSS-Tricks
• 15 Different scrollbar configurations

If you are talking about the scrollbar that automatically appears on a div with overflow: scroll (or auto), then no, that's still a native scrollbar rendered by the browser using normal OS widgets, and not something that can be styled(*).

Whilst you can replace it with a proxy made out of stylable divs and JavaScript as suggested by Matt, I wouldn't recommend it for the general case. Script-driven scrollbars never quite behave exactly the same as real OS scrollbars, causing usability and accessibility problems.

(*: Except for the IE colouring styles, which I wouldn't really recommend either. Apart from being IE-only, using them forces IE to fall back from using nice scrollbar images from the current Windows theme to ugly old Win95-style scrollbars.)

You can stablish specific toolbar for div

div::-webkit-scrollbar {
width: 12px;
}

div::-webkit-scrollbar-track {
-webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
border-radius: 10px;
}

see demo in jsfiddle.net