I don’t think this is possible.

Only way would be to have the birthdays element and have a picture of a birthday cake in its own element as an image.

Or set the whole background of the birthdays element to the cake

Hi @baris.halici ,

I just try whether we can do this. You can achieve your birthday surprise using below steps.

1. First I created a image using paint. Then saved the image in a shared location where any one of your company have the access. 

1. Then created a List element (Size will matter. Here I used 2x1) and added the data source who has the birthdays. Then as mentioned in the below red arrow pasted the shared location URL with to the “Custom Image URL:” section. 
   Ex: \\sanmlk1\S\image\cake.jpg

1. Now you can add the created element to the Lobby page.

Hope this will help you to create a birthday lobby element :)