child_style.css

/*
 Theme Name:     child
 Theme URI:      https://child-theme.org/
 Description:    child Theme
 Author:         Christian Mehlmauer
 Author URI:     https://child-theme.org/
 Template:       twentyfourteen
 Version:        1.0.0
*/

@import url("../twentyfourteen/custom.css");

.site,
.site-header {
 max-width: 100%;
}
.site-content .entry-header,
.site-content .entry-content,
.site-content .entry-summary,
.site-content .entry-meta, .page-content {
 /* Original max-width: 474px */
 max-width: 80%;
}
img {
  border: solid black 2px
}