It happened that one of our users has tried to extend a theme in Internet Explorer. Yes, we know that IE is not supported but people sometimes forget about such things.
Well, instead of opening a new extended theme or - better - throwing an error message, Limesurvey has opened the default theme and modified it a bit before noticing that there was a problem with that.
I've reproduced this behaviour, it opens the default theme regardless of which theme I try to extend.

I don't know if there will be a solution in the future, but I think that at present if there was a way prevent at all the IE browser from opening the theme editor page would be good enough.