Change wording from "No stock" to "Out of Stock"

Any text in HikaShop can be overridden as explained there:
www.hikashop.com/en/download/languages.html#modify

For the font color, you can add some CSS in the front end CSS file of HikaShop via the Display tab of the configuration, like that:

.hikashop_product_no_stock{ color: red; }

The front end CSS file. Be careful, I said the Display TAB of the configuration, not the Display menu which is displayed on all HikaShop back end pages.

No but make sure that you save the configuration and refresh your browser cache.

I see that you're still using the front end default CSS. Make sure that the custom one you created is selected and save again your configuration.