No I didn't remove the \ . The weird thing is that it is indeed missing and also when I tried to add it now in front of f069 as it should be, as you indicated it is not saved.

on:
survey.hc-barometer.gr/index.php/544526

see:
survey.hc-barometer.gr/upload/templates/test/css/template.css

let's see if it's a bug removing the \ on saving.

Here, changing css/template.css line 4 randomly from padding-bottom: 10px; to padding-bottom: 9px does not remove the slash in front of f069.

Just clicking save at the template editor (without making any changes) removes instantly the \. That seems to be the problem. So I edited the file directly from cPanel and managed to save the \ back to its place. So I will have to be editing the files directly to not mess \ again.