They have removed the option to add your own color themes. Has anyone figured out how to do this now?

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

Picture of screen