Custom CSS for Woo Emails

Not directly, no. You’d need to override the email templates with your own custom ones. There’s a starter guide available at https://docs.woocommerce.com/document/template-structure/

You could also look into using a 3rd party plugin like:

• https://www.ads-software.com/plugins/receiptful-for-woocommerce/
• https://codecanyon.net/item/email-customizer-for-woocommerce/8654473

Thanks so much for your response Riaan. I finally found a free plugin that worked perfectly: Decorator – Woocommerce Email Customizer and was able to easily adjust colors, padding, etc. with it.