Common Elements

If you look at /dijit/themes/tundra, you can see that a theme is just a bunch of CSS and images:

themes/
   tundra/
       tundra.css	<-- all the CSS for all the widgets
       images/
          checkbox.gif	<--- all the checkbox and radio button images
          fader.gif	<--- background image referenced by tundra.css

The foreground images are located in the images directory (along with background images) and are referenced from the widget via CSS rules (via the background-image property of a dummy node).

The tundra.css file has rules like:

/* GeSHi (C) 2004 - 2007 Nigel McNie (http://qbnz.com/highlighter) */ .geshifilter {font-family: monospace;} .geshifilter .imp {font-weight: bold; color: red;} .geshifilter .kw1 {color: #000000; font-weight: bold;} .geshifilter .kw2 {color: #993333;} .geshifilter .co1 {color: #a1a100;} .geshifilter .coMULTI {color: #808080; font-style: italic;} .geshifilter .es0 {color: #000099; font-weight: bold;} .geshifilter .br0 {color: #66cc66;} .geshifilter .st0 {color: #ff0000;} .geshifilter .nu0 {color: #933;} .geshifilter .re0 {color: #cc00cc;} .geshifilter .re1 {color: #6666ff;} .geshifilter .re2 {color: #3333ff;} .geshifilter .re3 {color: #933;} .geshifilter .re4 {color: #933;}
.tundra .dojoButton { ... }

We've been using Tundra in all of our examples, but to use an alternate theme for all widgets on your page, do:

/* GeSHi (C) 2004 - 2007 Nigel McNie (http://qbnz.com/highlighter) */ .geshifilter {font-family: monospace;} .geshifilter .imp {font-weight: bold; color: red;} .geshifilter .kw1 {color: #b1b100;} .geshifilter .kw2 {color: #000000; font-weight: bold;} .geshifilter .kw3 {color: #000066;} .geshifilter .coMULTI {color: #808080; font-style: italic;} .geshifilter .es0 {color: #000099; font-weight: bold;} .geshifilter .br0 {color: #66cc66;} .geshifilter .st0 {color: #ff0000;} .geshifilter .nu0 {color: #cc66cc;} .geshifilter .sc0 {color: #00bbdd;} .geshifilter .sc1 {color: #ddbb00;} .geshifilter .sc2 {color: #009900;}
<style type="text/css">
        @import "http://o.aolcdn.com/dojo/0.9.0/dijit/themes/themename/themename.css";
        @import "http://o.aolcdn.com/dojo/0.9.0/dojo/resources/dojo.css"
    </style>

Then add a theme name to the <body> element, like:

/* GeSHi (C) 2004 - 2007 Nigel McNie (http://qbnz.com/highlighter) */ .geshifilter {font-family: monospace;} .geshifilter .imp {font-weight: bold; color: red;} .geshifilter .kw1 {color: #b1b100;} .geshifilter .kw2 {color: #000000; font-weight: bold;} .geshifilter .kw3 {color: #000066;} .geshifilter .coMULTI {color: #808080; font-style: italic;} .geshifilter .es0 {color: #000099; font-weight: bold;} .geshifilter .br0 {color: #66cc66;} .geshifilter .st0 {color: #ff0000;} .geshifilter .nu0 {color: #cc66cc;} .geshifilter .sc0 {color: #00bbdd;} .geshifilter .sc1 {color: #ddbb00;} .geshifilter .sc2 {color: #009900;}
<body class="themename">

The reason the tundra theme requires a class=tundra on the body tag (or some other tag) is because the rules all list a hierarchy like ".tundra .dojoButton".