This error has been reported once:
An exception has been thrown during the rendering of a template ("variable @default-box-shade is undefined: failed at
box-shadow: <a class='mention' href='https://github.com/default'>@default</a>-box-shade;line: 3036"). plugins/CoreUpdater/templates/runUpdaterAndExit_welcome.twig 8 using PHP 7.0.24
Maybe the file containing that definition was not correctly updated?
Would say so too. I think this can be closed. Feel free to re-open if needed.