themes.php

• URL: /wp-admin/themes.php
• Capability required: switch_themes or edit_theme_options (and related caps like install_themes, delete_themes, update_themes).
• What it does: Displays installed themes, handles theme activation, deletion, auto-updates, and links to the Customizer/Site Editor.
• Key hooks: theme_auto_update_setting_template.

theme-install.php

• URL: /wp-admin/theme-install.php
• Capability required: install_themes (plus upload_themes for upload tab).
• What it does: Theme installer UI (search, featured, upload) and installs themes from WordPress.org or zip uploads.
• Key hooks: install_themes_tabs, install_themes_pre_{$tab}, install_themes_{$tab}.

customize.php

• URL: /wp-admin/customize.php
• Capability required: customize (and edit_post for the changeset).
• What it does: Boots the Customizer controls iframe, loads changeset, and prints Customizer controls UI.
• Key hooks: customize_controls_init, customize_controls_enqueue_scripts, customize_controls_print_styles, customize_controls_print_scripts, customize_controls_print_footer_scripts, customize_controls_head.

custom-background.php

• URL: /wp-admin/custom-background.php
• Capability required: edit_theme_options (via admin bootstrap).
• What it does: Legacy background image/color settings page for classic themes.
• Key hooks: (none in file; standard admin hooks apply).

custom-header.php

• URL: /wp-admin/custom-header.php
• Capability required: edit_theme_options (via admin bootstrap).
• What it does: Legacy header image settings page for classic themes.
• Key hooks: (none in file; standard admin hooks apply).

site-editor.php

• URL: /wp-admin/site-editor.php
• Capability required: edit_theme_options.
• What it does: Boots the Site Editor (full-site editing) for block themes.
• Key hooks: enqueue_block_editor_assets, site_editor_no_javascript_message.