Currently there’s no way to add custom CSS to popups (both the popup widget and the poupup in the portfolio widget). It would be awesome to be able to apply custom css there)
Folks, our former Wishlist portal, where our users shared their requests and suggestions, was transferred to this forum. You’re most welcome to vote, add new ideas, and leave your comments here — we surely will consider them all! 
Original Votes: 3
Hi friends,
Great news — Custom CSS is now available for Popup! 
See where you will find this option:
- Go to your widget Settings tab
- Find Custom CSS field and add any code you need
Feel welcome to check the widget out in our Popup online demo! (Popup - Add responsive Popup widget to your website (free and fast))
And we’ll be happy to hear your feedback in the comments below 