This doesn’t seem to be directly related to Contact Form 7, but sounds more like a general question regarding styling of emails.
Styling html-emails is a very flaky science. If you’re old enough to know what I mean: it’s like styling websites in the early 2000s. You need to test in a lot of different email clients, and the results will be different every time.
You would be best off just keeping your email as simple as possible. Instead of relying on CSS try to change the HTML. For example instead of outputting <ul><li> in your emails just output <span> or plain text if you want to show it inline.
However, if you feel like you want to find out first hand how hard it is to style e-mails here’s a post that can get you on your way: https://stackoverflow.com/questions/4829254/best-practices-for-styling-html-emails
