Joplin version: 1.1.1 and later
Platform: Windows, possibly others too
OS specifics: Windows 10 Home Single Language version 1809
v1.1.1:

v1.0.245:

I expected the colors to remain the same. This could be a feature and not a bug, but I don't think so since I believe the old colors look better, and since it is not listed as a feature in the release notes of v1.1.1.
The new release versions included a move to a new editor component which did not have the same themes available. You can customize the colours however you want with the userchrome.css file.
Take a look at my comment on the forums for some information about customizing further.
If you're really stuck changing the colours yourself, feel free to drop a comment below and I can try to help you out.
Just to clarify: Does this issue being closed mean that dark theme syntax highlighting will not be part of Joplin in the foreseeable future?
@m-ad Take a look at the link posted by CalebJohn above. In that forum, a user called dpoulton posted some CSS that you can place in the userchrome.css file for more colorful syntax highlighting. As for whether there are plans to make this the default in the future, I don't know.
Just to clarify, there are no current plans to change the default syntax highlighting.