Help changing header navigation from underline to strikethrough

Site URL: https://sparrow-greyhound-p8nb.squarespace.com/

Hi there, 

I am having trouble editing my site header. Firstly I was having trouble changing the menu nav underline colour to white, to match the rest of the text. However I would prefer for the line to be a strikethrough instead (when that page is selected) - is this possible? 

div#topNav a:hover {
   text-decoration: line-through;
}

the above does not seem to work

Site is currently being built & a domain is yet to be assigned. Passcode for the site is 530

Would really appreciate any help.

Thank you!

Also - It seems any changes I make within the Custom CSS field have no effect on the site - I fear it may be ineffective as I have made an error, however it does not give me any indication of doing so? 

struggling here! If you can shed any light & help me and my feeble coding mind, we would appreciate it.  

Hi. What is password?