Hi,
Just updated my Manager Desktop Version. Noticed that the themes of an Invoice, Quotes are missing. I was testing the India GST and was looking to change the Invoice theme
Regards,
Prasanna
Depending on when you last updated, you may be seeing the effect of a change in how themes are handled. Themes are no longer selected within the program, but copied from the web site (so there can be more without making the program too large). See this release note: [17.6.0] New workflow for custom templates - #96 by Citusgler.
With the change, if you do not have any custom themes created (by yourself), the option of switching does not appear. If that does not answer your questions, be more specific.