You can do this by using the following ways.
- Use the ThemeKey module
ThemeKey allows you to define simple or sophisticated theme-switching
rules which allow automatic selection of a theme depending on current
path, taxonomy terms, language, node-type, and many, many other
properties. It can also be easily extended to support additional
properties exposed by other modules.
Add the following code to your settings.php
$theme_urls = array(
array('/admin','garland'), // Drupal already supports admin theme, anyway
array('/users','fancy_user_theme'),
array('/forum','forums_theme'),
);
foreach ( $theme_urls as $info) {
if (strpos($_SERVER['REQUEST_URI'], $info[0]) === 0) {
$conf['theme_default'] = $info[1];
}
}