From what version of Desktop edition did you switch to Cloud version?
there had been recent changes regarding the themes. you could read about it here
you can set the theme for each invoice and it will remember your choice. so when you go back to view that invoice it will show the theme you selected. but there is no option to set as a default.
i believe the invoice title is automatically changed to Tax invoice when the default tax rates available in Manager is selected. If its a custom tax code then you will have to make use of custom theme.
The title change occurs for tax codes where it is known that regulations require that change. If you have some other in-built tax code where that doesn’t happen, and you have a reference that shows it should, provide that. If you just want to change it for your own reason, read this Guide: https://www.manager.io/guides/7877.
You also cannot change the Plain theme. You can copy it from the Themes web site (it’s the first one) and modify it to become something else. But you will always have Plain as a theme.