Welcome to the Shopify community!
and Thanks for your Good question.
Please share your site URL,
So I will check and provide a solution here.
Thanks for store URL.
i have check this it's not possible to minor CSS code it can be done more customization so
It can be done by doing some code customization. Please take the help of a developer and hire Shopify Expert or me
We have the option of changing the Quantity selector on the Boundless product page to include + & - buttons.
Besides the other features we of our Boundless product template.
You can take a look and see it from here:
Regarding the customization of the template, you will be able to do that easily from the theme dashboard, after the setting up.
I sent you more details about that via a private message.