I think you are not targeting the element correctly. Try this:
#mainmenu {
display: block;
}
#mainmenu ul.nav-primary {
list-style-type: none !important;
background-color: rgba(0,0,0,0.25) !important;
padding: 5px !important;
border-radius: 8px !important;
-moz-border-radius: 8px !important;
-webkit-border-radius: 8px !important;
}
#mainmenu ul.nav-primary li {
display: inline-block !important;
background-color: rgba(0,0,0,0.25) !important;
margin-right: 5px !important;
height: 32px !important;
padding-top: 8px !important;
border-radius: 8px !important;
-moz-border-radius: 8px !important;
-webkit-border-radius: 8px !important;
}
#mainmenu ul.nav-primary li a {
text-decoration: none !important;
color: #fff !important;
margin: 8px 14px 0 14px !important;
}
Try replacing the code you’ve mentioned above or adding it at the bottom of file ↓
http://steven.doig.com.au/wp-content/themes/transparent/style.css