My self-created theme no longer works under PHP 8. I would like to change it so that it continues to work. Currently only the navigation is displayed.
I have deactivated all plugins.
I have tried to solve this via the help page, but I can't find the error(s).
I have provided the theme as a ZIP.
I would be grateful for any help.
The page I need help with: https://www.johanneskroening.de/