feat: enable dark mode auto switch without reloading

[Crocmagnon]

What does this PR change? What problem does it solve?

Don't rely on JS to toggle dark mode when disableThemeToggle: true.

Was the change discussed in an issue or in the Discussions before?

Nope

Side note: there's some duplication which could probably be avoided, however I'm not familiar enough with hugo & go templating.

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
87.9% Duplication

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