Add a season-specific class to the body tag.

stranger_studios_season_body_class.php

<?php
/**
 * Add a season-specific class to the body tag.
 *
 */
function stranger_studios_season_body_class( $classes ) {
	$months_winter = array( 1, 2, 11, 12 );
	if ( in_array( date( 'm' ), $months_winter ) ) {
		$classes[] = 'body_winter';
	}
	return $classes;
}
add_filter( 'body_class', 'stranger_studios_season_body_class' );