[How To?] I want to add custom background on index of the forum

[Noter33]

Hi ,
I want to add a custom background on index of the forum using CUSTOM PROFILE FIELDS , but for that I need to go in index and add this css code

body{
  background: url({$userfields['fidX']});
  
}

When I try to put something in my own profile field the whole page body is getting white and is not changing .
Did I invoked wrong the url ? I shouldn't use ?

{$userfields['fidX']}