theme
override a default .tpl.php file in a custom module
to basically use a template file in a custom module use the hook_theme_registry_alter(&$theme_registry) like this
Unset core stylesheet in a custom theme
by using use hook_css_alter() in template.php
usefull image functions
how to get an image path or theme a preset image ?
override a default .tpl.php file in theme
To override a default .tpl.php file (to NOT use the page.tpl.php file for instance but page--mysnippet.tpl.php)
Change the theme
Programmatically change the theme on runtime by implementing hook_custom_theme()
Theme and render
display some content using drupal_render() and theme (this is the official way to use theme function)