Before changing the visual design (otherwise known as the "theme", or "skin") of your Plumi site it is helpful to understand how a Plone site is put together. This tutorial on plone.org breaks down the main display elements in a Plone site, and shows you how to control and customize the appearance of each element.
The most basic way to change the layout of your site is to rearrange, add or delete portlets as described in the previous chapter.
One of the easiest ways to change the look of your Plumi site is to change the CSS. A tutorial on how to change the CSS by using the Firefox developer tool Firebug, in conjunction with the Zope Management Interface (ZMI) can be found here.
More documentation on changing the visual design of your Plumi site can be found on plone.org here, including the following topics.
A great reference guide to theming Plone:
Advanced theming topics: