Hello,
Add the next code in Theme Options > Custom CSS:
h2.wc-bacs-bank-details-heading,
h2.woocommerce-order-details__title {
font-size: 20px;
font-weight: 600;
}
h3.wc-bacs-bank-details-account-name {
font-size: 18px;
font-weight: 700;
}
Regards