Thank you so much you help me to solved this problem Thank you again,
I was wondering if i can add more than one class in a div but today i learned that i can add more and how to do that :), i use :
if(function_exists('register_sidebar'))
register_sidebar(array('name' => 'sidbar',
'before_widget' => '<div class=" block_cat block_arc ">',
'after_widget' => '</div>',
'before_title' => '<h1 class=" block_cat block_arc ">',
'after_title' => '</h1>'
));