It happens with almost every theme.Does it happen with any theme or a specific one? It's definitely not normal, but if it's a specific theme, it might not be fully compatible with the S9. Also, make sure you don't have any UI-altering apps on top of themes that might conflict with things.
The Samsung theme or the Nova theme (icon pack)?No, Nova shouldn't interfere with the UI, only maybe icon sets and wallpapers. Try clearing Theme's cache and see if that helps. I haven't seen any reboots on mine with the themes I've tried.
So, it has only rebooted once after the Samsung theme was applied and that's it?
Actually, a reboot by the user, (not a random reboot )is good after a theme is applied. BUT, it's not normal for the phone to reboot. Is this a theme from the Samsung store?
I would uninstall Nova if you no longer use it. You can always backup any current desktops on Nova and uninstall and reinstall later if you want to use it again.
Thanks for the suggestion.I agree, they shouldn't interfere so something is wrong with your theme engine. To check if it is Nova, for some reason, try switching to the stock launcher and applying a theme you know reboots the phone.