You must have a copy of the following files from a computer running High Sierra (10.13) or lower In this example I used files from Sierra (10.12)
If you would like the stationery in the original mail application then skip step 1, disable SIP, and continue with the original application, SIP can be reenabled after. Make copies of everything. Instead of "replace" add a
.orig to the file extension.
Duplicate the Mail Application
Right click on the new Mail Application and go to
ComposeWindow.toolbar with the
ComposeWindow.toolbar from the copied files
Go to Finder>Go To Folder>
Mail folder from the copied files to this folder
Open the Mail application and compose a new email
Right Click on the Toolbar and select
Drag the icon labeled
Show Stationery to your toolbar and hit done
Show Stationery button in your toolbar
Note: The toolbar item DOES work with dark mode but the Stationery Panel does not.