Theme Change from root

[Adrian101]

Hi there, 

I have a store running on Presta 8.1.27 and a theme is causing that admin cannot be found or it gives this error ( Fatal error: Uncaught Error: Call to undefined function imageavif() in /www/wwwroot/presta/classes/ImageManager.php:700 )

I searched the forums and the internet on how to change the theme back to classic theme from either the database or from the root folders themselves. 

config/settings.inc.php "  define('_THEME_NAME_', 'default') " does not work as its deprecated as of 1.7 or so it says. Ive tried going directly to the theme database ps_shop and change the theme there as someone suggested but that just gave me another error

Any guidance into how to change the theme back to classic theme would be appreciated

Just to clarify.. I cannot get into admin, it says the page cannot be found, front end works, some pages works but some gives the above error

(More info, not sure if its relevant I moved the store to another domain name and also from apache2 server to nginx, its not server related or atleast I dont think so as I have moved a wordpress site from apache to nginx and its working properly)

[JBW]

Change the theme_name in table ps_shop should work. If there is another error you have to work on that.