If you're using Moodle 1.9: http://docs.moodle.org/en/Theme_basics
If you're using Moodle 2+: http://docs.moodle.org/en/Development:Themes_2.0
If you're using Moodle 1.9: http://docs.moodle.org/en/Theme_basics
If you're using Moodle 2+: http://docs.moodle.org/en/Development:Themes_2.0