ThemeManager OEM was designed to work for people without Object Desktop and for those with as well. If you already have OD with Theme Manager installed, ThemeManager OEM will just add a theme. In thiscase, DarkStar will have been added to any themes/suites that you already had. You should have received a "ThemeManager OEM has detected" message during install that let you know this.
You can go to the ThemeManager interface, select "Classic Desktop" to disable all aspects of Darkstar and then select a WindowBlinds skin as you normally would. That will leave you with the Windows default icons, wallpaper, etc. Or you can just change the WindowBlinds skin, which will leave you the Darkstar icons, etc.
If you still have questions or for any reason it doesn't work as expected, email support@stardock.com