Themes are the decorative arrangements of your site allowing you to create the graphic interface of the site. With the hundreds (or indeed thousands) of themes and their variants available for WordPress there is probably a theme out there that you would consider a perfect match for the content of your blog. This is also useful for advanced bloggers who can manipulate a close-matching theme into a custom work for their site.
You can find an overview and links to theme lists in the WordPress Codex section. Some of the themes you find are stupendous, others are not-so-great. When you are in a theme-list you can download the zipped files for the theme, which you can then upload into you wp-content/themes folder, unzip and they become ready for use via the Themes tab under the Presentation tab in the Administration screen (check the Available Themes section on the page).
To switch from the Current theme to an available them simply click the new theme and that will instantly take-over the site.
One rather exciting development has been the Sandbox theme – essentially a base theme from which others (even non-designers) can be built. This has seen many good designs and over time you may be able to build a theme yourself using the Sandbox – I believe it is much easier to build in Sandbox than starting from scratch:
The Sandbox is a theme for themers. It has the ability to be easily skinned, so beginners will feel comfortable styling it since they only need to know CSS. More experienced designers will drool at the rich semantic markup and profusion of classes, dynamically generated by a few functions. Read more here.
The page also includes downloads for the Sandbox theme. As with all other themes you simply place the zipped file into your wp-content/themes/ directory and unzip the theme. This will allow you to use the themes created within the Sandbox environment (see Sandbox for a live demonstration of Sandbox themes. Once you see a theme you like simply download the zip file, upload it to your wp-content/themes/ directory and unpack. It will then become one of the available themes in your Presentation > Themes tab.
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