Removing date on blog posts with code in the Hayden template

I am trying to edit my blog to not show dates for blog posts. 
I have watched several videos and understand that with my version I am unable to do it through settings and can only edit this through code. 
I found some old blog posts specific to the Hayden template and took the codes from a suggestion there, putting them at the bottom in the CSS section as described, but it simply won't work.   

Can anyone advise what I am doing wrong? 

Thanks.
 

.blog-date {
    display: none;
}