chore: update banner faster under theme change

[realguse]

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This PR refactors the banner part in the readme in order for it to detect the theme faster as illustrated below.

Old (the banner clearly lags behind in updating):
https://github.com/user-attachments/assets/ed2cc76c-7f18-4644-978f-0e6facbfc4f6

New (no log when switching themes):
https://github.com/user-attachments/assets/9eb4144b-5d3a-4dd2-91e9-27840e549fa2

The lag problem in the first example will get slightly better when switching theme for the second time, but the same amount of lag appears when refreshing and then switching.

[changeset-bot]

⚠️ No Changeset found

Latest commit: d53d670

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15320-svelte.vercel.app/

[Rich-Harris]

Thank you but this is an undesirable change — it means that everyone visiting https://github.com/sveltejs/svelte would download both banner.png and banner_dark.png