Add wp-theme-<name> and wp-child-theme-<name> classes to body tag

[nirajgiriXD]

Description

This PR introduces new classes to the body tag in WordPress themes. The classes wp-theme-<name> and wp-child-theme-<name> (if child theme) are added, where <name> represents the sanitized name of the active theme and child theme, respectively.

Changes Made

• Added wp-theme-<name> class to the body tag for the current theme.
• Added wp-child-theme-<name> class to the body tag for the current theme (if child theme).

Screenshots

• For twentytwentyfour theme

• For a child theme named childtheme having twentytwentyfour as parent theme

Trac ticket: https://core.trac.wordpress.org/ticket/19736

[github-actions]

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Props nirajgirixd, poena.

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[github-actions]

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Test this pull request with WordPress Playground.

[carolinan]

If these class names are intended to help plugins with theme specific styles, they also need to be present in the Site Editor.
(They are already available in the Customizer preview)