Style custom columns in admin panels (especially to adjust column cell widths)

I found a solution that works for me!

I dropped this code in functions.php :

add_action('admin_head', 'my_column_width');

function my_column_width() {
    echo '<style type="text/css">';
    echo '.column-mycolumn { text-align: center; width:60px !important; overflow:hidden }';
    echo '</style>';
}