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if you are modifying these themes for personal use, feel free to hack away. If you plan on releasing these themes to the public, the respectful thing to do is ask the original author for permission.
A theme is comprised of several files working together and when modifying one you will probably work with these files
theme.html - the base theme file, utilizes all other listed files
theme_blockcenter_c.html - defines how center-aligned center blocks appear
theme_blockcenter_l.html - defines how center-aligned left blocks appear
theme_blockcenter_r.html - defines how center-aligned right blocks appear
theme_blockleft.html - defines how blocks in the left sidebar appear
theme_blockright.html - defines how blocks in the right sidebar appear
style.css - css stylesheet used in most browsers including IE and Opera
styleNN.css (optional) - css stylesheet used in gecko (Netscape) based browsers
styleMAC.css (optional) - css stylesheet used for the Mac platform
If you are not familiar with CSS, I would highly recommend checking out
http://www.w3schools.com/css/ which will explain the basics. Also there is a commented stylesheet for the default theme what each style rule controls:
http://wiki.xoops.org/wakka.php?wakka=DefaultThemeCSS