How to Apply One Theme on Multiple WordPress Sites
If you want a consistent design across your entire WordPress network, you can apply one theme to multiple WordPress sites. For example, you may have a custom theme specially made for the main site in your network, with added features to display network-wide content. It may make sense for you to create a theme that is used on all sites within your network (other than your main site) that is created from the same design, with the same color scheme and graphics to provide that consistent look; if consistency and network branding is your goal.
When the network is enabled, available themes are shared among all sites. If a change to a theme file is made, every site in the network using that theme experiences the change because only one copy of the theme is being served. When a theme is enabled, it appears in the administrator’s Dashboard on the Manage Themes page (which you can access only if you are the administrator by clicking Dashboard→Appearance→Themes). Users can choose to activate this theme so it displays on the front side of the site. You must activate a theme for use across all sites in a network by clicking the Network Enable link under the Theme name in the Network Admin Dashboard (by clicking Network Admin and then clicking Themes).
The main network site could have 20 different themes installed in the main WordPress installation; however, if you have not enabled them for use across the entire network, then your site administrators will not see network-disabled themes in their Dashboards and therefore, will not be able to use them on their site.
If a consistent network design is what you’re after, you will run into a few troubles with the WordPress network because, by default, no matter what themes you have activated, the default WordPress Twenty Ten theme is the theme that gets activated whenever new sites are created within your network. It would be nice for WordPress to provide you with a global setting in the Network Admin Dashboard that would allow you to assign the default theme to every site that is created; however, that is currently not the case, unless you want to edit some code in the WordPress configuration file.
There is, however, a wonderful plugin that adds a simple item in the Network Admin Settings options called Default Theme. It gives you the option to assign a default theme to be the theme displayed on newly created sites.
The Default Theme plugin is not a free plugin, unfortunately; rather, it is available from the development group at WPMU Dev. To access the plugin, you do need to purchase membership to their site.
The WPMU Dev membership gives you access to hundreds of WordPress network-related plugins and themes for one annual membership. After you have your WPMU Dev membership; you can begin downloading any one of hundreds of plugins and themes for your WordPress network.