✨ feat(theme-switcher)!: respect theme_default when JS is enabled

[welpo]

‼️ BREAKING CHANGE ‼️

If you had default_theme set and want the colourscheme to follow user OS preferences (as it did before this PR), set default_theme = "" or comment it out on your config.toml.

Summary

This PR ensures the default_theme specified in config.toml is respected on first load.

Related issue

Resolves #223.

Changes

• Updated initializeTheme to prioritize the default_theme setting from the config.toml file, ensuring that this setting is respected on the first website load.
• Modified themeSwitcher script to include a defaultTheme variable and updated the theme application logic to align with the new initial theme setting approach.
• Enhanced the themeSwitcher script's functionality for resetting the theme, now prioritizing the default_theme setting.

Accessibility

This update maintains the existing accessibility features, including semantic HTML, ARIA attributes, and keyboard navigation compatibility.

Type of change

• UI/UX improvement (enhances user interface without altering functionality)
• Documentation update

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Checklist

• I have verified the accessibility of my changes.
• I have tested all possible scenarios for this change.
• I have updated theme.toml with a sane default for the feature.
• I have made corresponding changes to the documentation:
  • Updated config.toml comments.
  • Updated theme.toml comments.
  • Updated "Mastering tabi" post in English.
  • (Optional) Updated "Mastering tabi" post in Spanish.
  • (Optional) Updated "Mastering tabi" post in Catalan.