Hey there.
The original developer is probably best on this as it’s his/her theme and they would be more familiar. ??
That said, looking at the file used for the icons it seems to miss Instagram, and then the CSS for G+ and Pinterest doesn’t seem to be there on your site.
You could add something like this to your CSS:
.default-googleplus:before{content: "\e00d";}
.default-pinterest:before{content: "\e601"}
.default-instagram:before{content: "\e601"}
The last one is using the same content as Pinterest. You will most likely want to change that, perhaps even use an icon:
https://findicons.com/icon/579976/instagram_16_black?id=583170
You should download and upload to your own site but as a quick example using their link:
.default-googleplus:before{content: "\e00d";}
.default-pinterest:before{content: "\e601"}
.default-instagram:before{content: url("https://findicons.com/files/icons/2779/simple_icons/16/instagram_16_black.png"); position:relative; top:3px;}
NOTE: You should not hotlink to their site, you should download the icon and upload to your site if you wish to use it.
You could add this to a child theme:
https://codex.www.ads-software.com/Child_Themes
https://premium.wpmudev.org/blog/create-wordpress-child-theme/
Or you could use a simple plugin like:
https://www.ads-software.com/plugins/custom-css-manager-plugin/screenshots/
https://www.ads-software.com/plugins/my-custom-css/screenshots/
https://www.ads-software.com/plugins/pc-custom-css/
https://www.ads-software.com/plugins/imporved-simpler-css/screenshots/
Or when the theme developer pops in he/she might have another suggestion. ??
Your website should now look like this:
https://monosnap.com/image/BWJ8ylFaLJfjLjVhOXlO5VdIKasElD
??
Hope this helps.