I am trying to make a small form, that just has a submit button, and on submission, it should toggle user meta for the WordPress user. The button exists on frontend after content of post, and has a action too.

<form action="" method="POST">
<input type="submit" name="test" id="test" onclick=""/><br/> </form>

The above was the html form code, i can assure that it echoes properly. Then i add the below line to trigger it. This is php

if (isset($_GET['dark'])){ 
testfun(); 
}

This calls a function called testfun, the function testfun is below:

function testfun(){
$user_id = get_current_user_id();
$theme= get_user_meta($user_id, 'theme', true);
if ($theme == 'dark') {
   update_user_meta($user_id, 'theme', 'light');
   }
   else{
   update_user_meta($user_id, 'theme', 'dark');
   }
}

And obviously this code doesn't work, and i fail to figure our why. I have an intuition that the way i gey user id, can be wrong, because rest all seems to be fine. Anyhow please help me with this code.

I basically want to create a button that for the current logged in user, updated the user meta key 'theme' and toggles it between values 'light' and 'dark'

And as far as I am concerned i couldn't find anything wrong in it.