They have told us that the answer is hidden in the past. It's a hint to use wayback machine. So I went to archive.org and searched for cyberange.io
In the search result, I saw that the site is live since 2016. 
I have checked every screenshot starting from 2016 one by one and found the flag on a screenshot of 28th April, 2018. 
I was looking for the theme owner and found '#Flatfy Theme by Andrea Galanti' at the bottom of the page.
So, our flag is:  flag{andrea_galanti}
Important: You have to submit the name in lowercase.
Thanks