You should be able to replicate that even in Word 365 (Desktop) by opening Word, going to Options, (or File then options if you have a document open) click the Save category on the left then check (or unchecking) the box for “Don’t show the Backstage when opening or saving files with keyboard shortcuts”.
In the latest version of Word 365 if I check that box I get the floating Save dialog. If I uncheck it I get the embedded version.
Thing is I’m pretty sure that option, or something very similar, has existed in the same place with every Word version since 2013 and perhaps even 2010. You can learn more about “Backstage” here.
I think the Circuit Board Background / Theme was introduced along with theming in either 2010 or 2013 and can still be done even in the latest versions of Word 365. You can find out how to do that here.
uninvitedguest