help: Update documentation on switching themes.

[sayamsamal]

As a follow-up to commit ad013a9, this updates the /help/dark-theme docs to describe the newer way to switch themes via the theme switcher, located in the Personal Menu.

This replaces the documentation on switching themes via "Personal Settings".

Fixes: #29550.

Screenshots and screen captures:

Before	After

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[zulipbot]

Hello @zulip/server-user-docs members, this pull request was labeled with the "area: documentation (user)" label, so you may want to check it out!

[sayamsamal]

@alya Does the updated documentation read well to you? The last point might be a bit on the lengthier side, and need splitting? I don't have much idea about it, so waiting for your feedback.

[alya]

Thanks! Our pattern is not to list out the options; I'll add a note about that here.

[alya]

Thanks! Our pattern is not to list out the options; I'll add a note about that here.

#29906

[sayamsamal]

@alya ready for another review.

[timabbott]

Merged, thanks @sayamsamal!