Yes you can customize nav bar using CSS, e.g. to put your nav bar on right side you can use: (insert in Settings / Advanced customization (css) )
#idNavigationContainer {float: right;}
#idPrimaryContentContainer {margin-left: 0px;}
To hide the Join Now page when logged in you can use the following script: (insert in Settings / Global javascript) - we have already done this for you!!!
<script>
if (!document.getElementById('idLoginUserName'))
{ var menuItem = document.getElementById('idMainMenuItem161946');
if (menuItem) {menuItem.style.display = 'none';}}
</script>
Jim Sympathy, Fluid Apricot