Functions like

• theme()
• hook_theme()
• drupal_render()
• drupal_find_theme_*()
• template_preprocess_HOOK()

are well known and widely used but have lots of pitfalls, infamous but still useful approaches, different performance issues or not widely used tricks, etc.

Element's properties like

• #after_build
• #field_prefix/suffix
• #post_render
• #pre_render
• #suffix
• #prefix
• #process
• #theme
• #theme_wrappers

can be used for different theme purposes, but every property/callback has its own purpose and messing it up brings problems and "miracle" behaviors.

Unfortunately back-end developers and front-end developers usually have some communication problems:

• "Put field description before field itself"
• "Add class here if ....."
• "Can you remove all unused HTML here"
• "I want full template for this particular form"

and I will show how it can be done only by front end developer from his custom theme.