Ubuntu is already coming with some of the themes which you can simply select and apply. The themes can give your desktop different look and feel. All the panels, icons and styles can be changed. Just to play around with the default themes go to
System -> Preference -> Appearance
Then a window will open where you can see many themes and by just clicking on the themes those will be selected. Also you can see a button called customize. If you click on that you can see many tabs for icons, background, windows border etc. Just select them and change the icons, windows border etc and you will see the change on your desktop.
You can also download themes and install it in your machine. You can get themes from Gnome-looks
You will be getting tar file. Now you have to open the appearance from System -> Preference . There you will see one button with name install. When you click it will lead you to browse the file from the location where you downloaded the tar file. Then it will get installed and apply the new theme. Also you can download icons from the same site and install those like this.
MAC Theme
You can get the icon theme for this from mac icon theme
You can even your desktop look like a MAC desktop. The first step is to get a panel with all the application icon on it.
You can achieve it by a simple customization of your panel. Either you can delete the current panel and create a new one or you can simply continue on the same panel. Anyway you have to make the old panel look like a new one. For that just delete all the things on the panel by right clicking and giving Remove from panel . After cleaning your panel or deleting and creating a new panel right click on the panel. And select the prorperties. In the Panel Properties change the size into 50 (default will be 25). Then unselect the expand (remove the tick). Now change the tab to Background from General . There select the Solid Color and move it to the left end, thereby the panel will become transparent.
Now you can add application to the panel . For adding right click on the panel and click on Add to Panel And add all the applications that you may need.
Nowif you change the icons from theme preferences you can see the icons changing. To make it look like MAC OS you may need to get the MAC icon theme from the above mentioned site. You can download the icon theme of mac and install it.
