Exercise 3: Why Block vs. Classic Theme Changes the Customization Interface — Possible Solution
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WHAT DEFINES A CLASSIC THEME, PER THIS CHAPTER
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Per this chapter's own table, a classic theme has its "page structure...
defined in PHP template files rather than editable blocks." The
overall site structure (header, footer, page layout) is written in PHP
code the Block Editor from WordPress Fundamentals 4 was never designed
to reach into or edit directly.

WHAT DEFINES A BLOCK THEME, PER THIS CHAPTER
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Per this chapter, a block theme is "built specifically for the Full
Site Editing capability... headers, footers, and full page templates
are all editable as blocks." Here, the entire site structure - not
just individual post/page content - is itself expressed using the same
block system already introduced in WordPress Fundamentals 4.

WHY THE INTERFACE HAS TO DIFFER AS A RESULT
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Per this chapter, "on a block theme, much of what the Customizer used
to handle moves into the Site Editor instead." The Customizer was
built to expose a fixed, predetermined set of settings (site identity,
colors, menus, homepage settings) precisely because a classic theme's
own PHP-defined structure isn't itself directly editable through blocks
- the Customizer had to work around that limitation with a curated
settings panel. A block theme removes that limitation entirely, since
its structure is already made of editable blocks, so it makes more
sense to expose that structure directly through the Site Editor rather
than through a separate, more limited settings panel.

WHY THIS ISN'T AN ARBITRARY UI CHOICE
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The interface a site owner sees is a direct consequence of what the
underlying theme actually allows: a classic theme's fixed PHP structure
can only be exposed through a curated set of Customizer settings,
while a block theme's block-based structure can be exposed and edited
directly and comprehensively through the Site Editor. The difference in
interface reflects a genuine underlying difference in how much of the
site can actually be edited as blocks in the first place.

WHY THIS WORKS AS AN ANSWER
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It defines both theme types precisely using this chapter's own wording,
explains the structural reason (PHP-defined vs. block-defined
structure) that forces two different customization interfaces to
exist, and clarifies that the difference in interface is a genuine
technical consequence rather than an arbitrary design decision.