This has nothing to do with ".hikashop_product_stock table tr, .hikashop_product_stock table td", so you should not add "display: block;" there.
Just copy and paste these following 3 lines as a separate section into your custom CSS file:
.hikashop_product_stock_count {
display: block;
}
This could be your template's "custom.css", indeed.
Alternatively, and as said before, you could create and maintain HikaShop's own frontend custom CSS file following the link I had also already given you:
www.hikashop.com/support/support/documen...the-display.html#css