Place this in functions.php of your theme or somewhere else it can be called. This adds a link to the WP Admin bar that will deep link you to edit a particular section of the theme customizer in the admin backend.
You can read more about this here
The trick is to link to /customize.php?autofocus[section]={your_section_here}. Some section names in default WP are:
- title_tagline (For Site Identity)
- colors (For colors)
- background_image (For Background Image)
So if you wanted to deep link to editing the site identity it would look like:
'href' => get_admin_url() . '/customize.php?autofocus[section]=title_tagline'