Trying to use a foreign key with Postgresql

I've already sent this request for help to the novice mail list but I
never get any answer. So I wonder if it was the good list for that kind
of question and I'm publishing it here.
(thousand of apologies if I'm wrong !)

I'm a beginner with postgres and I'm trying to understand how I can
workaround the FOREIGN KEY mechanism within Postgres.

I've read that it's not supported yet and we can use a trigger.

So I created:
CREATE TABLE "tbl_article" (
"i_article" serial PRIMARY KEY,
"repere" text,
"refmedia" int4 NOT NULL,
"theme" int4 NOT NULL,
"mot_cle1" int4 NOT NULL,
"mot_cle2" int4,
"mot_cle3" int4);

and:
CREATE TABLE "tbl_theme" (
"i_theme" serial PRIMARY KEY,
"theme" text NOT NULL UNIQUE);

then:
CREATE FUNCTION chk_itheme_proc(int4) RETURNS int4
AS 'SELECT COUNT(i_theme) FROM tbl_theme
WHERE i_theme = $1;'
LANGUAGE 'sql';

and I created a trigger:
CREATE TRIGGER chk_itheme_trig BEFORE INSERT OR UPDATE ON tbl_article
FOR EACH ROW EXECUTE PROCEDURE chk_itheme_proc ('theme');

but I get the message:
ERROR: CreateTrigger: function chk_itheme_proc() does not exist

If I do:
SELECT theme FROM tbl_article
WHERE chk_itheme_proc(5) = 1; -- i_theme goes from 1 to 12

theme
-----
11
(1 row)

If I do now:
SELECT theme FROM tbl_article
WHERE chk_itheme_proc(15) = 1;
theme
-----
(0 row)

So I can see that the function is available ....

I'm stuck with this issue and I wasn't able to find any help on
documentations.

Is there somebody who could help me ???

Many thanks in advance,
PhiL