Hi there,
for the Quantity Buttons on the single product and the cart page, you can add this CSS:
.woocommerce form.woocommerce-cart-form .quantity.buttons-added .minus,
.woocommerce form.cart .quantity.buttons-added .minus {
border-top-left-radius: 5px;
border-bottom-left-radius: 5px;
}
.woocommerce form.woocommerce-cart-form .quantity.buttons-added .plus,
.woocommerce form.cart .quantity.buttons-added .plus {
border-top-right-radius: 5px;
border-bottom-right-radius: 5px;
}