Customizing WordPress

Most "look and feel" customizations are accomplished via editing the theme's style.css file.  

If possible, use a child theme:

https://codex.wordpress.org/Child_Themes
https://codex.wordpress.org/CSS

I am using Twenty Fifteen Theme, so how do I do this?

The instructions are in the WordPress Codex link I posted above:

https://codex.wordpress.org/Child_Themes#How_to_Create_a_Child_Theme

Basically, you create a new folder in wp-content/themes.  Then create a style.css file in that new folder and add the following code to the file:

/*
 Theme Name:   Twenty Fifteen Child <-- CHANGE THIS TO WHATEVER
 Theme URI:    http://example.com/twenty-fifteen-child/ <--- CHANGE THIS TO YOUR FOLDER NAME
 Description:  Twenty Fifteen Child Theme
 Author:       John Doe
 Author URI:   http://example.com <--- CHANGE THIS TO YOUR SITE
 Template:     twentyfifteen
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  http://www.gnu.org/licenses/gpl-2.0.html
 Tags:         light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
 Text Domain:  twenty-fifteen-child
*/

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If you want the theme to use the base TwentyFifteen style, create a new file in your child theme folder called functions.php and add this code to it:

<?php 
add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
function theme_enqueue_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );

}

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That imports the base CSS from the TwentyFifteen parent.  You can also copy/paste the CSS from the parent to the child instead.

Is the Theme URI important,l as I'm developing it on localhost,

I did the steps above and see "Twenty Fifteen Child" blank theme, but now what?

Is the Theme URI important,l as I'm developing it on localhost,

Not really.

I did the steps above and see "Twenty Fifteen Child" blank theme, but now what?

Make the child theme your active theme and add/change the CSS as desired.

What I meant regarding the URI, is that the address will change. For example, I changed the installation folder and all CSS is disabled.

Regarding the child theme, now I have activated the CSS file is empty. What do I add in order to reduce the gap between the side bar and content?

Sorry, I needed to go out of town for emergency.

I added #sidebar and it adjusted a sub-section of the menu, but I cannot find the primary container, which hold sidebar.

From the image attached, I would like the left-hand bar (white part) to be 1/3 of what it is now, and the content to be right next to it glued with no gray area.

This is my css so far

/*
 Theme Name:   Twenty Fifteen Child
 Theme URI:    http://localhost/wordpress0/wp-content/themes/twentyfifteen-child/
 Description:  Twenty Fifteen Child Theme
 Author:       John Doe
 Author URI:   http://example.com
 Template:     twentyfifteen
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  http://www.gnu.org/licenses/gpl-2.0.html
 Tags:         light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
 Text Domain:  twenty-fifteen-child
*/
#sidebar{
	width: 150px;
}
article{
	position: relative;
	top: 0px;
	left: 150px;
}

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Capture.JPG

No definite solution.  Will re-post at later date.