If you are talking about creating a new theme from an original yes the easy facility is gone. one now needs to program the theme file now which is not easy.
If you want to ImPORT a release 6 theme that you created a while ago that feature is still there.
menu tools settings appearance themes (Import on lower right of screen)
If you used default in release 6 your old ones will be in a directory similar to this
C:\Users\your_user name\AppData\Roaming\eM Client\themes
If your c:\user folder is not visible you will need to change the folder options and un check the hide system folders.
If you want to try mine Classic view where color is now a light blue with red lines between mail
https://www.dropbox.com/s/s4z95afl8n4ujmq/Classic%20DarkerBlue.zip?dl=0
Picture of screen
