WP_Customize_Themes_Section example

'description' => (
                    '<p>' . __( 'Looking for a theme? You can search or browse the WordPress.org theme directory, install and preview themes, then activate them right here.' ) . '</p>' .
                    '<p>' . __( 'While previewing a new theme, you can continue to tailor things like widgets and menus, and explore theme-specific options.' ) . '</p>'
                    ),
                    'capability'  => 'switch_themes',
                    'priority'    => 0,
                )
            )
        );

        $this->add_section(
            new WP_Customize_Themes_Section(
                $this,
                'installed_themes',
                array(
                    'title'      => __( 'Installed themes' ),
                    'action'     => 'installed',
                    'capability' => 'switch_themes',
                    'panel'      => 'themes',
                    'priority'   => 0,
                )
            )
        );