Josh Pollock

Use this plugin to create a demo site for your themes, or use it to allow users to customize the presentation of your site.
The JP Theme Bar Plugin adds a theme switching bar to the bottom of your site, perfect for theme preview sites. The settings page for the plugin lets the end user choose which themes to add, as well as set the colors for the theme bar. You can see it in action on this site.
New in version 0.0.3 -> Themes will update theme mods when changing themes allowing for individual theme settings to be previewed properly.
This plugin is based on the <a href="http://iainjmccallum.com/wordpress/live-demo-theme-bar/" rel="nofollow ugc">IJM Theme Switcher Bar v2.0</a> by <a href="http://www.iainjmccallum.com/" rel="nofollow ugc">Iain J McCallum</a>
IMPORTANT- The plugin <a href="https://wordpress.org/plugins/theme-test-drive/" rel="ugc">Theme Test Drive</a> by <a href="http://www.prelovac.com/vladimir/" rel="nofollow ugc">Vladimir Prelovac</a> must be installed and activated in order for the theme switching to work.

JP Theme Switcher Bar

Adds a theme switcher/ theme demo bar to the bottom of your site to allow users to switch the theme they see on your site.

Eric Marden

thecuriousfrog

This plugin is the equivalent of a theme switcher for styles applied to the K2 theme. It allows your visitors to re-skin your site from a list of K2 styles that you select.
The default K2 style to be applied to a given post or page can be specified using a parameter added to the URL, post or page meta data, or an overall site default.
Style selections made by the user are persistent for the duration of their visit and override default values, however they can be overridden using a URL parameter.
<h3>Uninstallation</h3>
<ul>
<li>If you wish to stop using the plugin but keep your configuration options then simply use the ‘Deactivate’ link on the WordPress ‘Manage Plugins’ screen</li>
<li>If you wish to completely uninstall the plugin then use the ‘Deactivate’ link followed by the ‘Delete’ link on the WordPress ‘Manage Plugins’ screen. This removes the plugin files and your configuration options</li>
</ul>
<h3>Usage</h3>
This plugin includes a widget that displays either a dropdown menu or an unordered list of the available styles. The title of the widget and the type of menu to use are set in the widget options.
The other plugin options are set using the K2 Style Switcher administration panel (‘Appearance’ > ‘K2 Style Switcher’).
The style to apply to a given post or page is chosen using one of the following methods:
<ol>
<li>A paramter can be appended to the page URL containing the relative path to the style, for example: <code>http://example.com/?k2ss_target_style=style/style.css</code></li>
<li>If a style has already been set in the PHP sessions variables then it will be applied to the post or page</li>
<li>A custom field called <code>k2ss_default_style</code> can be added to the page or post. The field value should be set to the relative path to the style, for example: <code>style/style.css</code></li>
<li>If none of the above methods have set the style then the default style specified in the K2 Style Switcher administration panel is applied</li>
</ol>
<h3>Configuration</h3>
Default Style 
This option sets the K2 style that visitors will see upon first landing on your page unless another style has been specified using one of the methods described in the Usage section. Once they select a style, they will see that style instead.
Default Style Picker Format 
This option sets the type of style picker that will be displayed when the <code><?php k2ss_style_picker(); ?></code> function is called with no parameter set:
<ul>
<li>Unordered List – Displays a list of links</li>
<li>Dropdown Menu – Displays a dropdown menu</li>
</ul>
If you opt to use the unordered list when displaying the style picker you can style the link to the currently selected style using the <code>.k2ss_selected</code> class. For example: <code>.k2ss_selected { border-left: 1px dashed #666; }</code>
Footer Template 
This option sets the text that is displayed in your site footer. This is re-written each time the page is loaded, meaning that it always displays information relevant to the style that the visitor is currently seeing.
For a list of the available macros click the option name (‘Footer Template’) on the administration panel. This will display the list of macros and the information they will be replaced with at runtime.
Display WordPress footer 
This option controls the visibility of the standard WordPress footer (‘Powered by WordPress and K2’). You may wish to hide this footer if you are placing the same information in the custom footer template.
Display feed links 
This option controls the visibility of the feed links footer (‘Entries Feed and Comments Feed’). You may wish to hide this footer if you are placing the same information in the custom footer template.
Display base styles footer 
This option controls the visibility of the active styles K2 footer (‘Styled with by ‘). You probably wish to hide this footer as any base styles that are active will probably be tweaks that do not need crediting.
Display page stats 
This option controls the visibility of the WordPress page loading stats footer (‘ queries. seconds.’). You may wish to show this information, it will appear just above the output of the custom footer template.
Available Styles 
This option displays all the styles currently available in your K2 styles directory. Select the styles you want visitors to be able to use. Note that styles are only shown here if they are not currently in use by K2.
Any styles that are set as active on the K2 options screen will be used as base styles. This means they will be applied in addition to either the default K2 Style Switcher style or the style that the visitor has chosen. This allows any custom CSS snippets to be maintained in one place rather than being added to each style.
<h3>Release Notes</h3>
<ul>
<li>No known issues are present in this version</li>
</ul>

K2 Style Switcher

This WordPress plugin allows site visitors to change the active style in the K2 theme.

JP Theme Switcher Bar vs K2 Style Switcher