If you’ve ever used Powershell to send HTML tables in Outlook containing CSS you’ve probably been disappointed of the outcome.
There is some archived documentation for Outlook 2007 that is still viable for Outlook 365 (https://msdn.microsoft.com/en-us/library/aa338201(v=office.12).aspx).

Basically the function accepts a csv and css file, hardcodes the css into the table and outputs a formatted HTML table that is compatible with Outlook.

I wrote this during my time @Xenit and the post should be found here
Read more on: https://tech.xenit.se/sending-css-formatted-tables-outlook/