I like to play around with themes and icons to make my desktop looking more lovely. Can I use the themes and icons also for Unity 3D/2D, or what do I have to look for to know when and how I have to use them?
What themes you can use with Unity depends on the version of Ubuntu you are running.
If you are running Ubuntu 11.10 or later, you can use the same themes you would use with GNOME 3 (or, more accurately, GNOME Shell). This is because Unity itself is based on GNOME 3 in those versions. Unity in Ubuntu 11.04 and earlier is based on GNOME 2, so it uses a different set of themes.
Icon sets should be completely compatible with any Ubuntu release under any GNOME-based desktop environment (including Unity). Also, there should be no theme differences between Unity 3D and Unity 2D.
You can get themes and icon sets from sites like gnome-look.org or deviantART. After you download your desired theme sets, see How do I change the theme? or How do I get and install more themes, icons, pointers for Ubuntu? for information on how to apply them.
First, install GNOME Tweak Tool from the Ubuntu Software Center.
Once you download a theme or icon set you'll have to extract the theme or icon set and put it in the right folder. Use