The Emerald Theme Manager is a tool to choose which Emerald window decorator theme you want to use. In order for this to work you will need to have the Window Decorations plugin enabled, and Emerald chosen as the window decorator.
To import themes, click on "Import..." and navigate to the .emerald file you wish to import. There are quite a few nice themes at [http://beryl-themes.org].
Select a theme from the list and your window decorations should change immediately. You can edit any theme easily, for instance to change the shadow or title font, but you can also go into more complicated aspects of customization, changing colors and opacities, etc.
Follow the instructions in the Repositories tab to fetch more themes from the internet. You may get "Error calling tar" a few times, but that seems to be normal as of now.
In the Emerald Settings tab you can change various global emerald settings such as the button fading options and titlebar double-click actions.