A wiki's design can be updated by any of a community's admins by using the Theme Designer. If you are not an admin and wish to help with your wiki's design, contact your local admins first.
If you would like help with creating a more customized design, you could try asking on the Community Central forums.
Finally, those with JavaScript and CSS knowledge can make more changes to their wiki's design. Just keep to our customization policy and you can tweak your wiki to make it just as you want it!