You got some things mixed up here – let me try to point you in the right direction.
Terminology
In WordPress, a theme is a set of files and functions that provide the frontend look for your Website. Most themes also include some specific functionality, but the main goal of a theme is to style your WordPress website according to your needs.
A WordPress website uses one theme, that is one of the most basic settings.
The plugins that you ased for are a way of altering the functionality of your Website.
WPSE Rules
WPSE is not about plugin recommendations, but a forum to ask specific how-to questions. Please refer to the Help Center to learn hw to ask a good question
A method of solving this problem
Nevertheless, even if your question is not asked in a good way, I would like to provide you with an idea for a solution.
Basically, you want to use different styls for different pages, and WordPress offers a bunch of possibilities to achieve just that.
Using different templates
A very simple solution would be to use different templates to achieve a different look for your pages.
If you have a page-boys.php and a page-girls.php in the root directory of your theme, you can create different layouts for these two.
Ading a body class
This one is more complicated, as you need to know which class your HTML-body should have before your content is processed. You could use a hook depending on some post_meta to achieve that. Place this function in your functions.php, and be sure to create the post meta data where needed.
add_filter( 'body_class', 'f711_your_boy_girl_meta_function' );
function f711_your_boy_girl_meta_function( $classes ) {
if ( get_post_meta( get_the_ID(), 'f711_your_boy_girl_meta', true ) == 'boys' ) {
$classes[] = 'boys';
}
return $classes;
}
Enqueueing another stylesheet
Basically the same as the body class, you can enqueue a different stylesheet, if some post_meta is set.
Conclusion
You got a little bit of work to do, but fortunately for you, this is not too hard to do, and it’s a great way to learn about how WordPress works.
And please feel free to come back with any detailled questions that may arise on your journey 🙂