Hi Philippe,
We made this Catch Box theme to take H3 tag for Widget title. This cannot be change from CSS. Also why do you need to change it.
If you really need to change then you need to create child theme and then create empty functions.php
file in your child theme and then add the following code
function catchbox_widgets_init() {
register_widget( 'catchbox_adwidget' );
register_sidebar( array(
'name' => __( 'Main Sidebar', 'catchbox' ),
'id' => 'sidebar-1',
'before_widget' => '<aside id="%1$s" class="widget %2$s">',
'after_widget' => "</aside>",
'before_title' => '<h2 class="widget-title">',
'after_title' => '</h2>',
) );
register_sidebar( array(
'name' => __( 'Footer Area One', 'catchbox' ),
'id' => 'sidebar-2',
'description' => __( 'An optional widget area for your site footer', 'catchbox' ),
'before_widget' => '<aside id="%1$s" class="widget %2$s">',
'after_widget' => "</aside>",
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>',
) );
register_sidebar( array(
'name' => __( 'Footer Area Two', 'catchbox' ),
'id' => 'sidebar-3',
'description' => __( 'An optional widget area for your site footer', 'catchbox' ),
'before_widget' => '<aside id="%1$s" class="widget %2$s">',
'after_widget' => "</aside>",
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>',
) );
register_sidebar( array(
'name' => __( 'Footer Area Three', 'catchbox' ),
'id' => 'sidebar-4',
'description' => __( 'An optional widget area for your site footer', 'catchbox' ),
'before_widget' => '<aside id="%1$s" class="widget %2$s">',
'after_widget' => "</aside>",
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>',
) );
}