<li class="price_red"><?php echo TEXT_BASE_PRICE . $products_price; ?></li>

in my main stylesheet.css file I've included:

#price_red {
color: red;
font-size: 12px;
}
Your syntax is wrong -


.price_red {
color: red;
font-size: 12px;
}

( # is for id, . is for class )