Apply CSS To Particular Files

Apply CSS To Particular Files

I've inherited an MVC web application that administers a small database.
For most or all CRUD operations, the creator used popup screens and ajax
to allow the user to edit items.
This may be a simple question, but all the font size in the popups is
smaller than desired. What's the most efficient way to simply increase the
font size for these files? They all begin with the word 'partial' if that
matters.
I tried
<style>
* {font-size:1.3em;}
</style>
at the top of each partial file, but when the user opens the popup, all
the main page text gets updated with the larger font. Thankyou for any
help, I hope I have been clear enough.