Fix: Write Mode / Zoom Out: Dark toolbar inconsistency

[Rishit30G]

What?

Closes #67409

Why?

This PR improves the compatibility between Write Mode and Zoom Out functionality, ensuring they work seamlessly together.

How?

This PR disables the write mode experiment when the site editor is zoomed out and by default selects the 'Design' mode so that user sees the toolbar in light mode

Testing Instructions

• Enable the write mode experiment (Admin sidebar > Gutenberg > Experiments)
• Open the site editor and edit a template
• Click the Zoom out button on the topbar
• Notice on Zoom Out, the write mode is disabled
• User can only see 'Design' Mode when zoomed out ( light theme )

Screenshots or screencast

Screen.Recording.2025-03-31.at.10.33.26.AM.mov

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: Rishit30G <rishit30g@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>
Co-authored-by: patil-vipul <vipulpatil@git.wordpress.org>
Co-authored-by: richtabor <richtabor@git.wordpress.org>
Co-authored-by: talldan <talldanwp@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.