Please add this to Theme Options => Custom CSS:
/* Quantity Outline Color */
.woocommerce .quantity .minus,
.woocommerce .quantity input.qty,
.woocommerce .quantity .qty-adjust .woocommerce .quantity .plus,
.woocommerce div.product form.cart .variations select,
.woocommerce .quantity .qty-plus,
.woocommerce .quantity .qty-minus,
.woocommerce .quantity .qty-adjust {
border-color: red;
}
/* Font Color */
.woocommerce .quantity input.qty {
color: red;
}
/* Icon Up/Down Color */
.woocommerce .quantity .qty-adjust a > i {
color: red;
}