No that is not possible, from the relevant documentation:
public void setTheme (int resid)
Set the base theme for this context. Note that this should be called
before any views are instantiated in the Context (for example before
calling setContentView(View) or inflate(int, ViewGroup)).
So you have to set the theme with setTheme() before calling setContentView(), after that it is impossible to change it. Your only option is to recreate the Activity.
As an aside: That you say that recreating the Activity is not an option seems weird, Activities have a well defined life cycle and by adhering to this life cycle it should be possible to recreate the Activity how ever often you or the Android OS wants to. In fact you should know that your Activity can be killed and recreated by the Android OS at any moment and this is in fact a normal and expected behaviour of Android. So that recreating the Activity is not an option seems to indicate a much deeper rooted problem in your apps design that will manifest itself anyway if not solved.