Theme location
In the standard Moodle distribution, all themes are placed in the theme/ directory, this leads to theme developers hardcoding this location into their themes. Theme developers should not take this for granted, and use the variables which moodle provides for this purpose.
Starting with 1.7, moodle will fully support changing the location of themes for use with the theme selector. Themes need to be devloped to ensure compatibility with this feature.
$CFG->themewww
The $CFG->themewww variable contains the web-accessible location of the theme directory. If it is not set by the site administrator it will be the default of $CFG->wwwroot . '/theme'. i.e. 'http://my.moodle.site/theme'.
$CFG->httpsthemewww
The $CFG->httpsthemewww (introduced in Moodle 1.7) variable contains the same information as $CFG->themewww with the correct http/https prefix. This should be used to provide proper operation of https protected pages.
$CFG->themedir
The $CFG->themedir variable contains the local location on the moodle server of the theme directory. If it is not set by the site administrator it will be the default of $CFG->dirroot . '/theme'. i.e. '/my/moodle/location/theme'.