Index: src/wp-includes/class-wp-customize-control.php
===================================================================
--- src/wp-includes/class-wp-customize-control.php	(revision 31958)
+++ src/wp-includes/class-wp-customize-control.php	(working copy)
@@ -1235,34 +1235,6 @@
 }
 
 /**
- * Customize New Theme Control class.
- *
- * @since 4.2.0
- *
- * @see WP_Customize_Control
- */
-class WP_Customize_New_Theme_Control extends WP_Customize_Control {
-
-	/**
-	 * Render the control content.
-	 *
-	 * @since 4.2.0
-	 */
-	public function render_content() {
-		?>
-		<div class="theme add-new-theme">
-			<a href="<?php echo admin_url( 'theme-install.php' ); ?>" target="_top">
-				<div class="theme-screenshot">
-					<span></span>
-				</div>
-				<h3 class="theme-name"><?php _e( 'Add New Theme' ); ?></h3>
-			</a>
-		</div>
-		<?php
-	}
-}
-
-/**
  * Widget Area Customize Control class.
  *
  * @since 3.9.0
Index: src/wp-includes/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 31958)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -1171,13 +1171,6 @@
 			) ) );
 		}
 
-		if ( ! is_multisite() && current_user_can( 'install_themes' ) ) {
-			$this->add_control( new WP_Customize_New_Theme_Control( $this, 'add_theme', array(
-				'section' => 'themes',
-				'settings' => 'active_theme',
-			) ) );
-		}
-
 		/* Site Title & Tagline */
 
 		$this->add_section( 'title_tagline', array(
Index: src/wp-includes/class-wp-customize-section.php
===================================================================
--- src/wp-includes/class-wp-customize-section.php	(revision 31958)
+++ src/wp-includes/class-wp-customize-section.php	(working copy)
@@ -348,9 +348,6 @@
 			<div class="customize-themes-panel control-panel-content themes-php">
 				<h2><?php esc_html_e( 'Themes' ); ?>
 					<span class="title-count theme-count"><?php echo count( $this->controls ) - 1; ?></span>
-				<?php if ( ! is_multisite() && current_user_can( 'install_themes' ) ) : ?>
-					<a href="<?php echo admin_url( 'theme-install.php' ); ?>" target="_top" class="add-new-h2"><?php echo esc_html_x( 'Add New', 'Add new theme' ); ?></a>
-				<?php endif; ?>
 				</h2>
 				<div class="theme-overlay" tabindex="0" role="dialog" aria-label="<?php esc_attr_e( 'Theme Details' ); ?>"></div>
 				<div id="customize-container"></div>