say_hello on "how to arrange the text width in the 2014 theme?"

hello good day

mamy thanks tor the hints.... creating a child theme -
the link you showed me goes to a child theme for the 2015 -

here all the necessary things are described. new techniques and best practices are described. i can use this written code - see below - in the theme 2015 and other themes accordingly

many many thanks"!!! i will use the code to create a child theme.

/*

 Theme Name:   Twenty Fifteen Child
 Theme URI:    http://example.com/twenty-fifteen-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
*/

one question regarding the changes you mentioned. do i add this code to the css ? file

The code that is controling the content width of my theme is:

.site-content .entry-header, .site-content .entry-content, .site-content .entry-summary, .site-content .entry-meta, .page-content {
margin: 0 auto;
max-width: 474px;
}

or to the index php ?

can you tell me where to add this piece of code!?
many thanks for any and all help