A theme defines the overall appearance of the intro. It is simply a way of grouping all the presentation files (styles and images) in one place that can be pointed at or switched as one.
To define a theme, extend your intro configuration using the
org.eclipse.ui.intro.configExtension and use the theme
element, as shown in the example below:
   <extension
         point="org.eclipse.ui.intro.configExtension">
      <theme
            default="true"
            id="org.eclipse.ui.intro.universal.circles"
            name="%theme.name.circles"
            path="$nl$/themes/circles"
            previewImage="themes/circles/preview.png">
         <property
               name="launchbarBackground"
               value="#a1c2cb"/>
         <property
               name="launchbarOverviewIcon"
               value="$theme$graphics/launchbar/overview16.png"/>
         <property
               name="launchbarFirststepsIcon"
               value="$theme$graphics/launchbar/firststeps16.png"/>
         <property
               name="launchbarTutorialsIcon"
               value="$theme$graphics/launchbar/tutorials16.png"/>
         <property
               name="launchbarSamplesIcon"
               value="$theme$graphics/launchbar/samples16.png"/>
         <property
               name="launchbarWhatsnewIcon"
               value="$theme$graphics/launchbar/whatsnew16.png"/>
         <property
               name="launchbarMigrateIcon"
               value="$theme$graphics/launchbar/migrate16.png"/>
         <property
               name="launchbarWebresourcesIcon"
               value="$theme$graphics/launchbar/webresources16.png"/>
      </theme>
   </extension>
Consult the schema documentation for details on all the available attributes relating to themes.
Each theme has a unique identifier, translatable name, preview image, and a path
to the root theme folder. The intro plug-in does not provide any UI for theme
manipulation. The only way to select a theme is via the preference
org.eclipse.ui.intro/INTRO_THEME in plugin_customization.ini. 
Theme support by itself does not make intro implementations theme-enabled.
Concrete intro implementations can choose to expose themes in a more substantial
way (and in fact Universal Welcome implementation does exactly that with the
General > Welcome
preference page.
Theme-enabled intro implementation must make all the references to style and
presentation resources using the $theme$ substitution variable. Absolute paths
for images, pages, styles, etc. will be computed by resolving the substitution
variable using the path of the currently active theme.