I investigated this for a half an hour. I found the code snippet required to generate the CSS but didn’t have the time to figure out how to inject the inline CSS to HEAD in order to get it working.
This is from 2015’s functions.php
:
function twentyfifteen_post_nav_background() {
if ( ! is_single() ) {
return;
}
$previous = ( is_attachment() ) ? get_post( get_post()->post_parent ) : get_adjacent_post( false, ”, true );
$next = get_adjacent_post( false, ”, false );
$css = ”;
if ( is_attachment() && ‘attachment’ == $previous->post_type ) {
return;
}
if ( $previous && has_post_thumbnail( $previous->ID ) ) {
$prevthumb = wp_get_attachment_image_src( get_post_thumbnail_id( $previous->ID ), ‘post-thumbnail’ );
$css .= ‘
.post-navigation .nav-previous { background-image: url(‘ . esc_url( $prevthumb[0] ) . ‘); }
.post-navigation .nav-previous .post-title, .post-navigation .nav-previous a:hover .post-title, .post-navigation .nav-previous .meta-nav { color: #fff; }
.post-navigation .nav-previous a:before { background-color: rgba(0, 0, 0, 0.4); }
‘;
}
if ( $next && has_post_thumbnail( $next->ID ) ) {
$nextthumb = wp_get_attachment_image_src( get_post_thumbnail_id( $next->ID ), ‘post-thumbnail’ );
$css .= ‘
.post-navigation .nav-next { background-image: url(‘ . esc_url( $nextthumb[0] ) . ‘); border-top: 0; }
.post-navigation .nav-next .post-title, .post-navigation .nav-next a:hover .post-title, .post-navigation .nav-next .meta-nav { color: #fff; }
.post-navigation .nav-next a:before { background-color: rgba(0, 0, 0, 0.4); }
‘;
}
wp_add_inline_style( ‘twentyfifteen-style’, $css );
}
add_action( ‘wp_enqueue_scripts’, ‘twentyfifteen_post_nav_background’ );