Pure styled CSS is an open and free styled CSS, and it provides a very wonderful skin builder.

Really like it, and share with everyone.

<apex:page standardStylesheets="false" showHeader="false">
    <apex:styleSheet value="http://yui.yahooapis.com/pure/0.1.0/pure-min.css"/>
    <div id="yui_3_9_1_1_1370668995942_3017" class="widget-preview-label">Using Pure Styled CSS</div>
        <form class="pure-form">
        <fieldset>
            <legend>Legend</legend>
            <input placeholder="Email" type="text"/>
            <input placeholder="Password" type="password"/>
            <label>
            <input type="checkbox"/> Opt in
            </label>
            <button class="pure-button" type="submit">Sign in</button>
        </fieldset>
    </form>

    <div>Default Form</div>
        <form>
        <fieldset>
            <legend>Legend</legend>
            <input placeholder="Email" type="text"/>
            <input placeholder="Password" type="password"/>
            <label>
            <input type="checkbox"/> Opt in
            </label>
            <button type="submit">Sign in</button>
        </fieldset>
    </form>
</apex:page>

 The screenshot:

(.... Find no way to insert image here ...)

Just copy & paste the code of the visualforce to an new apex page, and you can find the screenshot.

Ref: http://purecss.io/