You easily can assign a different theme in selected WordPress posts and pages.
Page Theme is a free WordPress plugin by Chris Ravenscroft. This plugin allows you to select a different theme for your individual blog posts and pages.
Installation & Usage:
Step-1 Install and activate it, then visit Dashboard-> Appearance -> Page Theme and enable Page Theme on Posts and Pages
Step-2 For assigning a theme to a blog Post, visit Dashboard -> Posts -> All Posts page.
Step-3 You will find a new theme column in your All Posts list.
Step-4 Click the Default Theme icon for selecting a different theme for your post.
Step-5 Click Set Theme button.
For selecting a theme for any particular page, visit Dashboard -> Pages -> All Pages list and set the theme from theme column.