When the core is updated, my guess is they also update the theme and branding information, regardless of whether or not those packages are being updated according to apt.
Could you give us the context of the warning? the surrounding text?
I'm guessing three possible options:
1) apt is checking the theme and branding packages in apt update
2) apt is trying to upgrade these packages while upgrading the core but it warns you that these related packages hashes are wrong
3) the installation script for the core checks the theme and branding packages but for some reason the hashes aren't working out (Thus, it's not really anything to do with apt)
Because I know someone else is probably going to hop in, here are the packages on the debian website:
https://packages.debian.org/sid/lxqt-branding-debianhttps://packages.debian.org/sid/lxqt-theme-debianI hope I was a help.