Themes in Office 2007 are collections of fonts, colors, shape effects, etc. that you can apply to a document. Word, PowerPoint and Excel all have the same 20 themes so you can make your documents look similar. You can find the pre-defined themes on Themes group within Page Layout tab.
Note: If you have saved a document in compatibility mode, the themes group will be grayed out. You need to convert the document from compatibility mode to apply a theme.
If none of the 20 pre-defined themes appeal to you, you can find additional document themes on the Office Web site here.