Solved using this code:
if ($product->is_type( 'variable' )){
// Get the available variations for the variable product
$available_variations = $product->get_available_variations();
// Initializing variables
$variations_count = count($available_variations);
$loop_count = 0;
// Iterating through each available product variation
foreach( $available_variations as $key => $values ) {
$loop_count++;
// Get the term color name
$attribute_color = $values['attributes']['attribute_pa_size'];
$wp_term = get_term_by( 'slug', $attribute_color, 'pa_size' );
$term_name = $wp_term->name; // Color name
// Get the variation quantity
$variation_obj = wc_get_product( $values['variation_id'] );
$stock_qty = $variation_obj->get_stock_quantity(); // Stock qty
// The display
$separator_string = " // ";
$separator = $variations_count < $loop_count ? $separator_string : '';
// echo $term_name . ' = ' . $stock_qty . $separator . '<br>';
if ( $stock_qty > 0 ) { echo '<style> body span.swatch.swatch-label.swatch-' . $term_name. '{opacity:1!important; background: #484848!important; color: #fff!important;}</style>' ;} else { }
}
}