MariaDB 10.9

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select version();

version()

10.9.2-MariaDB-1:10.9.2+maria~deb11

create table themes (id int, variable text, value text);
insert into themes values (1,'city','Chicago'),(1,'state','IL'),(1,'zip',60000);

Records: 3  Duplicates: 0  Warnings: 0

SELECT GROUP_CONCAT(
  `value`
  order by case variable when 'city' then 1 when 'state' then 2 else 3 end
  SEPARATOR ""
) citystzip
FROM themes
WHERE `id` = '1' AND `variable` IN ('city', 'state', 'zip')

citystzip

ChicagoIL60000