Hi Flourishing,
a template for Sankey diagrams is a good idea if you wish to produce similar diagram layouts repeatedly. In the templates you can prepare entries with their colors, unit types and units, process shapes (color and layout), arrow options and graphical elements (such as your company logo).
Simply prepare your diagram style the way you like it. Then use 'Save As', but rather than saving the diagram as a .sankey file, save it as .santem file (Sankey template file). You will have to enter a file name, and subsequently a name for the template. This is how you later choose your template, e.g. "My Preferred Style". Note that it is not required to actually save visible diagram elements: you can do the settings, then delete all arrows and nodes, before saving to a template file. Pretty much like with a Word template, where you don't need text, but just set your styles...
If you save the .santem file in the default template folder (C:\Users\<USERNAME>\Documents\e!Sankey\<VERSION>\templates) it will be listed with its name in the "New Diagram" section at the top left immediately.
If you set the diagram as the default template by setting the path in the options dialog it will be shown in bold at the first position in the "New Diagram" template list. See screenshot below.
Template files are also described in the e!Sankey User Manual (via Menu Help).