Unable to change font sizes in css

Hi,

I have some troubles to change some codes in css. I don't know if the problem is with the child theme or wrinting code css.

Let's see if I did well the process.

I created a child theme ->exposito-child

Then I created a style.css with this content:

/*
Theme Name: expositio-child
Theme URI: expositio-child
Description: expositio-child
Author: Wpshower
Author URI: http://expositio.wpshower.com/
Template: expositio
Version: 0.1
.
Licencia, texto, lo que sea que quieras puedes escribirlo aquí ...
*/
@import url("../expositio/style.css");

Then I created functions.php with this content:

add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles() {
wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
wp_enqueue_style( 'child-style', get_stylesheet_uri(), array( 'parent-style' ) );
}

And then, I tried to add some code:

body {
font-family: 'Open Sans', sans-serif;
font-size: 12px
}

And it changes the font but the size was changed in some parts of the website, like captions. But not the site-title, h1, etc.

Then I tried to change it separately:

site-title {
font-size: 50px;
}

Also I tried to not write down any size in the body secction (font-size: 12px) and it doesn't work neither!!!!!
How should I do it?

The theme used is EXPOSITIO, and there is no any place to contact to the developers (the forum website doesn't work: http://wpshower.com/ )

Thank you!