Need to add some styling tweaks to your WordPress site?
In day yore, we would access the theme customizer by going to Appearance -> Customize in the Word Press admin interface. This still works on the majority of sites, but some modern themes support the "Full Site Editor". The full site editor is similar to the Gutenberg block editor. Some of us may prefer the classic editor over the block editor. We may prefer the legacy customizer over the full site editor. One advantage of the legacy theme customizer is that it allows developers to enter custom css via the “Additional CSS” interface. For block-based themes, this interface may be hidden.
To access the legacy customizer use the url pattern below.
https://mydomain.com/wp-admin/customize.php
Read an in depth article about the legacy theme customizer.
Read about the difference between the classic customizer and the block based full site editor here.