Transparent background in post content

What it looks like is the style.css file and the functions.php files were copied over from the parent theme. That’s not the right way to set up a Child Theme.

Can you replace all of the code of your functions.php file with this:

<?php

?>

Looking at your child theme style.css now and it looks like you have the entire twenty Fifteen CSS added to it which is not needed at all.

This is all that should be in your child theme style.css (unless you’ve made other CSS changes)

Change the child theme name, author etc to whatever you want.

/*
Theme Name: Blank Twenty Fifteen Child theme
Theme URI: http://yoursite/yourtheme
Description: A child theme of 2015 default WordPress theme.
Author: Your Name
Author url: http://yoursite.com/
Version: 1.0
Tags: black, blue, white, fixed-width, custom-header, theme-options
Template: twentyfifteen
*/

/* transparent post background */
.hentry {
	background-color: transparent !important;
}

This is all that should be in your functions.php (unless you have other functions you need)

<?php

//Import parent styles add other stylesheets if necessary.
function wpchildthemes12122_enqueue_child_styles() {

	wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );
}

add_action( 'wp_enqueue_scripts', 'wpchildthemes12122_enqueue_child_styles' );

Here’s a bit of reading to understand how child themes work and what to add and not add to them. http://codex.wordpress.org/Child_Themes