How to check if a theme is active?

I’d like to be able to check if the twentytwelve theme is active. I know if I was checking for an active plugin I’d do something like:

$active_plugins = apply_filters( 'active_plugins', get_option( 'active_plugins' ) );
if ( in_array( 'plugin-folder/plugin-folder.php', $active_plugins ) ) {
    //do stuff
} else {
add_action( 'admin_notices', 'create-a-notice' );
}

What’s the proper way to check if a theme is active so I can run a function for that theme?

You can use wp_get_theme:

<?php
$theme = wp_get_theme(); // gets the current theme
if ( 'Twenty Twelve' == $theme->name || 'Twenty Twelve' == $theme->parent_theme ) {
    // if you're here Twenty Twelve is the active theme or is
    // the current theme's parent theme
}

Or, you can simply check if a function in twentytwelve exists — which is likely less reliable; a plugin, or even another theme, could declare twentytwelve_setup, for instance.

<?php
if ( function_exists( 'twentytwelve_setup' ) ) {
   // Twenty Twelve is the current theme or the active theme's parent.
}