Can’t style custom menu

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