fix: Theme styles supersede default styles

[dcalhoun]

What?

Ensure theme styles supersede default styles.

Why?

Fix CMM-825.

When enabled, theme styles should take precedence over default styles.
Otherwise, it negates the "Apply theme styles" feature altogether.

How?

Relocate default global CSS variables so they are only loaded when theme
styles are disabled. Otherwise, due to the load order of stylesheets,
these default variables overrode theme style variables.

Testing Instructions

1. Install and activate the Kanso theme for your site.
2. Enable the "theme styles" for the experimental editor.
3. Open the experimental editor.
4. Verify the font size is correct for the post title and placeholder paragraph
   text.

Accessibility Testing Instructions

N/A, no navigation changes.

Screenshots or screencast

Before	After