404 background image

Could you please help me in how to change the 404 error background image?

Do I need to modify in template.css here??:
________
.error-page #gkPageContent {
background: url('../images/header_bg.jpg') no-repeat center top;
background-size: cover;
________

Thanks in advance,
Giacomo

Could you please post an url to your site?

I put the website online for you:
www.giacomopoleschi.com

Sound weird but it seems that if I try to modify template.css by using the built in editor in the admin side or by uploading by ftp (i.e. by changing for example the image in url('../images/headers/404_header_bg.jpg') ), after reloading a page on my browser, if I open again the file template.css in some way I loose my changes and I've back the original version of the file.:
________
.error-page #gkPageContent {
background: url('../images/header_bg.jpg') no-repeat center top;
background-size: cover;
________

I'm lost.
Thanks.

I tryed also to use override.css but I confirm you that I don't know how, after uploading by ftp my modified override.css file and after double checking it, If I reload the page http://www.giacomopoleschi.com/?itemId=99999 on the browser, all the files below are overwritten with the original version (so the changes i made are lost):

print.css
override.css
offline.css
mobile.css
template.css
tablet.css
small.tablet.css
small.desktop.css

Are those files overwritten on the ftp server?
Perhaps this is some weird hosting issue?
I have just checked your override.css and it is empty (usually there are few "commented out" lines at the beginning).

I got it!
For some reason the option "Recompile LESS" was ENABLED in Advanced settings-Template style, in Template Manager.
For this reason, the .css files where regenerated at each page reload.

Now it works with the override.css file modified.

Best,
Giacomo

Are those files overwritten on the ftp server?
Perhaps this is some weird hosting issue?
I have just checked your override.css and it is empty (usually there are few "commented out" lines at the beginning).

Thanks for the info.
Is there anything else I can help you with regarding this topic?