I kept having to hunt through wp-includes/class-wp-customize-manager.php to see what the default sections in the theme customizer were registered as.
To save you the trouble, here they are:
title_tagline
[gist id=”e182d64246dd579b089836cbe4473808″ file=”customizer-default-sections.php” lines=”3-6″ show_line_numbers=”0″]
colors
[gist id=”e182d64246dd579b089836cbe4473808″ file=”customizer-default-sections.php” lines=”8-11″ show_line_numbers=”0″]
header_image
[gist id=”e182d64246dd579b089836cbe4473808″ file=”customizer-default-sections.php” lines=”18-22″ show_line_numbers=”0″]
background_image
[gist id=”e182d64246dd579b089836cbe4473808″ file=”customizer-default-sections.php” lines=”24-28″ show_line_numbers=”0″]
nav
[gist id=”e182d64246dd579b089836cbe4473808″ file=”customizer-default-sections.php” lines=”30-35″ show_line_numbers=”0″]
static_front_page
[gist id=”e182d64246dd579b089836cbe4473808″ file=”customizer-default-sections.php” lines=”37-42″ show_line_numbers=”0″]
If you’re not sure how to add options to these default sections, give Otto’s post a read.