在html中添加将发送到电子邮件的css

I have created a method that will send an email with information to customers. However, the email looks awful cus there is no style to it. I can't apply the style to the email for some reason. I have tried to google it, and there is a lot on how to solve this in code behind, but that's not my issue. I must place the css code in the Html body, since it must be displayed for the client when he opens the email. I have created a method that will send an email