Add style to body based on url

for your home page:
On index.php or your home page custom file use:

if (is_home()){
  echo 'hi home!';
}

for any other page except 404:
use the file page.php or the custom one:
for example the link is www.xyz.pt/chat/

if (is_page('chat')){
  echo 'hi page!';
}

for page 404:

if (is_404()){
  echo 'hi error 404';
}