To reference a CSS file within an SVG file refer to the following example code.

<?xml-stylesheet href="styles_austria_1.css" type="text/css"?> 

Here's what an SVG CSS file looks like.

.str1 {stroke:#0093DD;stroke-width:16;color:#0093DD}
.str2 {stroke:#0093DD;stroke-width:12}
.str3 {stroke:#0093DD;stroke-width:10}
.str4 {stroke:#0093DD;stroke-width:6}

.str0 {stroke:#DA251D;stroke-width:55;color:#DA251D}
.str6 {stroke:#BB90BB;stroke-width:44;color:#BB90BB}
.str5 {stroke:#0093DD;stroke-width:4}
.str7 {stroke:#1F1A17;stroke-width:13}

.fil0 {fill:none}
.fil2 {fill:#1F1A17}
.fil1 {fill:#C4E5FA}
.fil3 {fill:#FFFFFF}

Source: Example for using CSS-Styles [carto.net]
Reference: Styling - SVG 1.1 - 20030114 [w3.org]

This HTML and CSS code shows how to customise buttons in a restful way. Source: http://particletree.com/features/rediscovering-the-button-element/

<div class="buttons">
    <button type="submit" class="positive">
        <img src="/images/icons/tick.png" alt=""/> 
        Save
    </button>

    <a href="/password/reset/">
        <img src="/images/icons/textfield_key.png" alt=""/> 
        Change Password
    </a>

    <a href="#" class="negative">
        <img src="/images/icons/cross.png" alt=""/>
        Cancel
    </a>
</div>

/* BUTTONS */

.buttons a, .buttons button{
    display:block;
    float:left;
    margin:0 7px 0 0;
    background-color:#f5f5f5;
    border:1px solid #dedede;
    border-top:1px solid #eee;
    border-left:1px solid #eee;

    font-family:"Lucida Grande", Tahoma, Arial, Verdana, sans-serif;
    font-size:100%;
    line-height:130%;
    text-decoration:none;
    font-weight:bold;
    color:#565656;
    cursor:pointer;
    padding:5px 10px 6px 7px; /* Links */
}
.buttons button{
    width:auto;
    overflow:visible;
    padding:4px 10px 3px 7px; /* IE6 */
}
.buttons button[type]{
    padding:5px 10px 5px 7px; /* Firefox */
    line-height:17px; /* Safari */
}
*:first-child+html button[type]{
    padding:4px 10px 3px 7px; /* IE7 */
}
.buttons button img, .buttons a img{
    margin:0 3px -3px 0 !important;
    padding:0;
    border:none;
    width:16px;
    height:16px;
}

Adding colour to the buttons - Colour is used like a traffic light system, green for go (positive), red for stop (negative, think about this for a moment), and blue which isn't a traffic light (neutral, a miscellaneous item)

/* STANDARD */

button:hover, .buttons a:hover{
    background-color:#dff4ff;
    border:1px solid #c2e1ef;
    color:#336699;
}
.buttons a:active{
    background-color:#6299c5;
    border:1px solid #6299c5;
    color:#fff;
}

/* POSITIVE */

button.positive, .buttons a.positive{
    color:#529214;
}
.buttons a.positive:hover, button.positive:hover{
    background-color:#E6EFC2;
    border:1px solid #C6D880;
    color:#529214;
}
.buttons a.positive:active{
    background-color:#529214;
    border:1px solid #529214;
    color:#fff;
}

/* NEGATIVE */

.buttons a.negative, button.negative{
    color:#d12f19;
}
.buttons a.negative:hover, button.negative:hover{
    background:#fbe3e4;
    border:1px solid #fbc2c4;
    color:#d12f19;
}
.buttons a.negative:active{
    background-color:#d12f19;
    border:1px solid #d12f19;
    color:#fff;
}