You certainly need to pay attention to the file extension - I've seen in other posts (a while ago) that some people posted "themes" that aren't just a theme but also override your settings (or at least parts of it). I think the Opus guys tend to remove those downloads in order to prevent unexpected calamities, but I suppose someone can still upload a "theme" that is more than just a theme. (I never install themes for this reason - and because I'm not hooked on fancy layouts anyway).
By the way, I wonder if there's a page that clearly identifies / describes the meaning of all possible file extensions eligible for import in Opus.
As long as it's actually a theme (.dlt file), it can only change cosmetic settings and is easy to undo (unless you opt out of backing up your current theme, although you can still always reset to the dark or light default themes).
(Config backups (.ocb files) made by other people are not themes and should be avoided in most situations.)