Yeah, but with the Discourse update I did today it shows a message on every page (for admins only of course). So, the topic is gaining momentum. 
[Admin-Hinweis] Theme ‘Topic OP and last reply for mobile’ enthält Code, der aktualisiert werden muss. (ID:discourse.static-viewport-initialization) (mehr erfahren)
And in the developer console:
[THEME 65 ‘Topic OP and last reply for mobile’] DEPRECATION NOTICE: Accessing
site.mobileVieworsite.desktopViewduring the site initialization can lead to errors and inconsistencies when the browser window is resized. Please move these checks to a component, transformer, or API callback that executes during page rendering. [deprecated since Discourse 3.5.0.beta9-dev] [deprecation id: discourse.static-viewport-initialization] [info: Designing for Different Devices (Viewport Size, Touch/Hover, etc.)]
@Lhc_fl any chance that you can fix that for this really great theme component? 