If you distribute a plugin, it must have a GPL compatible license. CC is not a source code license and is not appropriate for plugins.
Themes are another matter. There is an informally acknowledged loophole (similar to the arrangement with Linux kernel drivers) whereby theme templates are not considered combined works of WP even though they call into WP API. Of course, informally acknowledged doesn't have much legal standing. CSS doesn't call into WP at all, so it won't be considered a combined work.
The GPL only kicks in if you distribute your changes. If you keep your plugins and themes to yourself, you are not obligated to show your source.
So, if you distribute a plugin, it needs to be GPL compatible. If you distribute a theme that is wholly your own, you can give it whatever license you want. For best legal safety, however, make the templates GPL compatible. The CSS can be left as proprietary.
Making the license under which you release your plugin or theme clear is a good thing. In the US and other countries, copyright is automatic. If you don't see specific license information, assuming "all rights reserved" is safest since you don't know the author's intentions.
The only thing officially sanctioned by WP (whoever that is) is the WP distribution itself, the whole of which is released under the GPL. Anything hosted at wp-plugins.org must be GPL compatible.
Read the GPL FAQ.
Standard disclaimer: IANAL nor do I play one on TV.