functions.php

<?php
/* highlight keywords */
function my_highlight_search( $text ) {
    if( is_search() ){
  	$keys = implode( '|', explode( ' ', get_search_query() ) );
		$text = preg_replace( '/(' . preg_quote ( $keys ) .')/iu', '<span class="search-term">\0</span>', $text );
	}
    return $text;
}

add_filter( 'the_title', 'my_highlight_search' );
add_filter( 'the_content', 'my_highlight_search' );
add_filter( 'the_excerpt', 'my_highlight_search' );