to change product detail page background color

Could you post a link to the page ?

There is no CSS for that. You will have to edit the file show of the view product via the menu Display->views and add an additional div around the whole code so that you can apply your background-color to it.

In the file show of the view product, you need to add something like this at the beginning:
<div style="background-color:#FFF">

And at the end:
</div>

You can do that modification in the file show of the view product via the menu Display->Views.
If you want to do the modification directly, it's in the file components/com_hikashop/views/product/tmpl/show.php but in that case you will loose your changes when you update hikashop.