MailStyle allows you to write the css for your html emails as you normally would, then writes the styles inline when you send your emails. It also makes sure that your image paths are absolute rather than relative.

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

Prevents you from data breaches and liability risks
Upgrade at your own pace
Works with modern Rubies

MailStyle: A HTML Email Plugin for Ruby on Rails | Purify Blog

Rails Long Term Support