In the Variation Table for WooCommerce, including the quick cart where users can change the quantity, this setting controls the hover button color of the quantity plus minus button, allowing you to customize it to match your site’s design and branding.

Here is the frontend show for the variation table for the WooCommerce single product page. After hovering over those buttons, the background color will be changed to what you want.

Here is the frontend show for the quick cart carousel in the WooCommerce shop page. After hovering over those buttons, the button background color will be changed to what you want.