How to filter Less on child theme

Hi,

I’m trying to change the default color for buttons in the button widget. I’m running vantage with a child theme, and thought I could do something like this in the child theme’s `functions.php`:

function ms_filter_sow_button_less( $less, $instance, $widget ) {
        $less = str_replace('button_color: #41a9d5;',
            'button_color: #cecece;', $less);

        return $less;
}
add_filter('siteorigin_widgets_less_sow-button', 'ms_filter_sow_button_less',
    10, 3);

But … nothing happens. Am I misunderstanding something?