WIP: adds a theme variable

create.sh

         --skip-email \
         --title=$PREFIX \
         --url=https://www.$PREFIX.localhost";
-docker exec --user www-data $PREFIX-web /bin/sh -c '
-    wp theme install twentyseventeen \
+docker exec --user www-data $PREFIX-web /bin/sh -c "
+    wp theme install $THEME \
         --activate \
-        --path=/var/www/html';
+        --path=/var/www/html";