Css for flash messages, which you can choose yourself, since they are usually defined in application.html.erb (by default there is no default definition for flash messages in rails).
For formatting, I would recommend using a gem, for example formtastic , which not only greatly simplifies the creation of forms, but also provides a standard css file. Thus, all the necessary tags are known (and you can rewrite them if necessary).
If, on the other hand, you look at ways to quickly launch your full layout, you can check web-app-theme or activo (which is even more complete).
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