Thanks for contacting Syncfusion support.
We will update the information about the Fusion theme in our documentation with our Volume 1, 2020 main release which is expected to be rolled out in the end of this month.
However, you can download this additional themes for our theme studio application.
Note: Fusion is not a built-in theme and we do not have any plan to include this as such.
We intimate you once the release is out and appreciate your patience until then.
Jesus Arockia Sankaran S