3年后，我用下面的shell脚本解决了这个问题：

#Getting list of outdated themes
/usr/local/bin/wp theme list --update=available --allow-root --path=/var/www | sed -n \'1!p\' | awk \'{print $1}\' > /tmp/theme.list

#Update them one by one
while read -r themename
do
    #Store the old version number of theme
        OLDVER="$(grep Version: /var/www/wp-content/themes/"$themename"/style.css)"

    #Download theme
        wget -q https://downloads.wordpress.org/theme/"$themename".zip -P /tmp/

    #Uncompress .zip file
        unzip -oq /tmp/"$themename".zip -d /var/www/wp-content/themes/

    #Store the new version number of theme
        NEWVER="$(grep Version: /var/www/wp-content/themes/"$themename"/style.css)"

    #Create simple log file of theme update
        [[ "$OLDVER" != "$NEWVER" ]] && echo "$themename" >> /tmp/success.list || echo "$themename" >> /tmp/failed.list

    #Remove downloaded theme file
        rm /tmp/"$themename".zip
done < /tmp/theme.list
rm /tmp/theme.list