Hi Mark, 
Thank you for report, we have fixed this issue and will update in next OS Property version - 2.8.5
By the way, there are some steps to solve the issue. 
1. Open file: components > com_osproperty > osproperty.php
Find
[code]$document->addStyleSheet('http://fonts.googleapis.com/css?family=Voltaire');
$document->addStyleSheet('http://fonts.googleapis.com/css?family=Oswald:700');[/code]
and change to 
[code]$document->addStyleSheet('//fonts.googleapis.com/css?family=Voltaire');
$document->addStyleSheet('//fonts.googleapis.com/css?family=Oswald:700');[/code]
2. Open files: 
[quote]components > com_osproperty > templates > default > js > css > style.css
components > com_osproperty > templates > theme1 > style > style.css
components > com_osproperty > templates > theme3 > style > style.css[/quote]

Find
[code]@import url(http://fonts.googleapis.com...[/code]
and change to 
[code]@import url(//fonts.googleapis.com[/code]

Thanks
Dam