diff -r 34716fd837a4 -r be944660c56a wp/wp-admin/includes/class-custom-background.php
--- a/wp/wp-admin/includes/class-custom-background.php	Tue Dec 15 15:52:01 2020 +0100
+++ b/wp/wp-admin/includes/class-custom-background.php	Wed Sep 21 18:19:35 2022 +0200
@@ -117,8 +117,10 @@
 
 		if ( isset( $_POST['reset-background'] ) ) {
 			check_admin_referer( 'custom-background-reset', '_wpnonce-custom-background-reset' );
+
 			remove_theme_mod( 'background_image' );
 			remove_theme_mod( 'background_image_thumb' );
+
 			$this->updated = true;
 			return;
 		}
@@ -126,8 +128,10 @@
 		if ( isset( $_POST['remove-background'] ) ) {
 			// @todo Uploaded files are not removed here.
 			check_admin_referer( 'custom-background-remove', '_wpnonce-custom-background-remove' );
+
 			set_theme_mod( 'background_image', '' );
 			set_theme_mod( 'background_image_thumb', '' );
+
 			$this->updated = true;
 			wp_safe_redirect( $_POST['_wp_http_referer'] );
 			return;
@@ -204,8 +208,10 @@
 
 		if ( isset( $_POST['background-color'] ) ) {
 			check_admin_referer( 'custom-background' );
+
 			$color = preg_replace( '/[^0-9a-fA-F]/', '', $_POST['background-color'] );
-			if ( strlen( $color ) == 6 || strlen( $color ) == 3 ) {
+
+			if ( strlen( $color ) === 6 || strlen( $color ) === 3 ) {
 				set_theme_mod( 'background_color', $color );
 			} else {
 				set_theme_mod( 'background_color', '' );
@@ -308,7 +314,7 @@
 		<?php endif; ?>
 
 		<?php $default_image = get_theme_support( 'custom-background', 'default-image' ); ?>
-		<?php if ( $default_image && get_background_image() != $default_image ) : ?>
+		<?php if ( $default_image && get_background_image() !== $default_image ) : ?>
 <tr>
 <th scope="row"><?php _e( 'Restore Original Image' ); ?></th>
 <td>
@@ -412,7 +418,7 @@
 	<div class="button-group">
 				<?php foreach ( $group as $value => $input ) : ?>
 		<label>
-			<input class="screen-reader-text" name="background-position" type="radio" value="<?php echo esc_attr( $value ); ?>"<?php checked( $value, $background_position ); ?>>
+			<input class="ui-helper-hidden-accessible" name="background-position" type="radio" value="<?php echo esc_attr( $value ); ?>"<?php checked( $value, $background_position ); ?>>
 			<span class="button display-options position"><span class="<?php echo esc_attr( $input['icon'] ); ?>" aria-hidden="true"></span></span>
 			<span class="screen-reader-text"><?php echo $input['label']; ?></span>
 		</label>
@@ -484,6 +490,7 @@
 		}
 
 		check_admin_referer( 'custom-background-upload', '_wpnonce-custom-background-upload' );
+
 		$overrides = array( 'test_form' => false );
 
 		$uploaded_file = $_FILES['import'];
@@ -581,6 +588,8 @@
 	 * @deprecated 3.5.0
 	 */
 	public function wp_set_background_image() {
+		check_ajax_referer( 'custom-background' );
+
 		if ( ! current_user_can( 'edit_theme_options' ) || ! isset( $_POST['attachment_id'] ) ) {
 			exit;
 		}