KGIII,Thanks to your very clear steps I've reproduced this on hirsute Lubuntu.
Further testing suggests that the crash will occur if at least one Libreoffice application is opened when Impress is opened AND then you immediately open Libreoffice Math and then go back and select the vivid theme.
While I agree we have a bug here, it is a super corner case to hit.
KGIII,Thanks to your very clear steps I've reproduced this on hirsute Lubuntu.
Further testing suggests that the crash will occur if at least one Libreoffice application is opened when Impress is opened AND then you immediately open Libreoffice Math and then go back and select the vivid theme.
While I agree we have a bug here, it is a super corner case to hit.