I am looking for a note taking app with white text on a black background that works with iCloud. Apps for Mac and/or iOS will work. Any recommendation?
You should expand your requirement to iCloud and Dropbox for following reasons.
iCloud is sandboxed to only certain apps. In other words, any notes an app X creates cannot be read and modified by app Y. Dropbox allows any apps that supports Dropbox to access your notes. You may like app X now but you may want app Y later.
iCloud syncing can be useful if you have multiple iOS devices, for things like settings and miscellaneous data (e.g., scratchpad).
I have tried over a dozen note taking apps that syncs over the cloud. The app that I keep going back to is Elements (http://www.secondgearsoftware.com/elements/). It has very clean yet functional UI and frigging fast.
I do have few wish list items though: (1) tagging and/or support for moving notes to different folder, (2) hide certain Dropbox folders (I linked to the root folder), (3) default font setting (Museo Sans is very pleasant but may not be for everyone), and (4) Markdown keyboard (it fully supports Markdown but I don't remember all the syntax).
There's actually a very beautiful app called iA Writer that has a focus typing mode. Though normally supposed to be used for actual full-blown blogs and such, there's no reason you couldn't go and use it for notes.
This is the description of the iCloud feature from http://iawriter.com:
Your Documents on all Your Devices
iA Writer comes with iCloud and Dropbox integration. Instantly, any text you write anywhere will be updated on all your devices.
iA Writer for Mac can be downloaded here.