Taken from The WordPress MU Manual and applicable to standard WordPress as much as WPMU!
Each WordPress theme is made up of a collection of templates which includes a stylesheet (style.css) and several PHP template files.
The stylesheet controls the formatting and layout of your blog theme in terms of colour, what the font looks like, where elements are position on the page and so on.
PHP (stands for PHP Hypertext Preprocessor) is the scripting language used by WordPress to create the blog pages. When a visitor goes to your blog to read content, a request is sent to your server. This request is received by the PHP programming language which obtains the required information from your MySQL database and displays it in the visitor’s web browser.
While you don’t need know how to code PHP a basic understanding of both HTML and PHP means you can make minor edits to themes using a text editor such as EditPlus. EditPlus is a text editor, HTML editor and programmer’s editor for Windows. While you could edit in NotePad, EditPlus offers many powerful features that makes it better suited for editing PHP.
For Mac Users try using Smultron; free Text Edit software designed for Macs.
Always save a backup of your themes on your hard drive so that you have the original files in case it is necessary to restore the original themes.
Editing the blog footer
A simple theme edit is to change the link in the blog footer from ‘Powered by WordPress MU’ to ‘Hosted by’ and the name of your domain. For example, all Edublogs themes are ‘Hosted by Edublogs’.
You use EditPlus3 as follows to edit the blog footer in all 100 themes contained in the Farms 100 big ones theme pack:
1. Download and install EditPlus3.
2. Launch EditPlus3.
3. Locate and open up the folder on your computer that contains the themes.
4. Now open up the footer.php in one of the theme folders. For example, 72 classes
5. Locate and highlight all the text that creates the link in the blog footer that says ‘Powered by WordPress MU’ then go to Search and select Find in Files.
6. Change file type to *.*, select Include Subfolders, locate your folder that contains all themes that you want to edit and then click Find
This will locate every file in the themes folder that includes the ‘Powered by WordPress MU’ (for the 100 big one theme packs it should locate 100 occurrences)
7. Now right click and select Open All to open up every footer.php file that contains ‘Powered by WordPress MU’
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8. Locate and highlight all the text that says ‘Powered by WordPress MU’ then go to Search and select Replace.
9. In the Replace with field add the new text and link you want to use. For example, Hosted by <a href=”http://edutags.net/”>Edutags.net</a>
10. Now select All open files and click on Replace All.
11. This should have now replaced ‘Powered by WordPress MU’ with your new text and link.
12. Now go to File and select Save All to save all the changes to the footer.php. Next upload the edited themes to your WPMU site using FileZilla.
Next up… languages :)