Hello Matthias,
(Note, Me[Leo] /Eric are different developers).
Let me explain how the theaming works, then I'll try to answer your question within that context.
One thing to understand is that "Dark Theme" is not an SWT concept, but a platform-ui concept.
SWT doesn't have a notion of light or dark theme. Instead it just returns whatever color it is given by the operating system at initialization. These will be the colors defined by OS theme.
Now the Platform.UI theming is dependent on user preference. The user may want to select a "Eclipse light theme" or an "Eclipse dark theme". If a "Eclipse dark theme" is selected, then Platform.UI manually passes colors to SWT and overrides OS defaults.
(But in addition, it asks SWT to use "dark" variants of the OS theme. But let's ignore this for now).
So via SWT alone, there isn't a way to deal with Eclipse theming changes, because SWT doesn't know about Eclipse theming. If you want your U.I to respect Eclipse theming, then you need to use Platform.UI defined colors and listen to changes to that color in user preferences. Now, if you're asking about changes in OS theming, then that's a different matter. (let me know if that's what you're after).