Custom styled Icons

You can always upload your icons as documents, then utilize a!documentImage() to display them.  If you have a specific use case, feel free to share it here.  

We also recently added a bunch more out-of-the-box icons in the rich text icon component. Even if you've used these in the past, it might be worth checking again to see if something new could work for you!

docs.appian.com/.../Appian_Icons.html

1127 icons, love it!!  I'll now be spending the remainder of my day looking for excuses to implement "dizzy" and "hotdog"

I would like to use them in a a!buttomWidget. How ? 

a!buttonWidget(
label: "Submit",
submit: true,
style: "PRIMARY",
icon : "???"
)

Appreciate your icons! However we need domain specific icons which we design ourselves.

For that you'll need to use one of the official icons.  A workaround that's sometimes usable is using the "card as a button" pattern where you take a Card Layout, fill it with whatever you want (including a document image), and make it clickable like a button.  There are obvious limitations to this approach, though.