feat: add theme modality support for light/dark themes

[rawkode]

I don't know if this is something the project is looking to adopt, but I forked and made this work for Ghostty for my own experiment and plan to add support for more modules.

Sharing for feedback and to hopefully upstream.

Related #447

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Add support for configuring separate light and dark themes that can be used by applications supporting automatic theme switching. This includes:

• New theme.light and theme.dark configuration options
• defaultTheme option to control fallback for single-theme apps
• Full backward compatibility with existing configurations
• Ghostty integration with automatic light/dark theme switching
• Documentation for theme modality configuration and migration

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• [x] I certify that I have the right to submit this contribution under the MIT license
• [x] Commit messages adhere to Stylix commit conventions
• [x] Theming changes adhere to the Stylix style guide
• [x] Changes have been tested locally
• [ ] Changes have been tested in testbeds
• [ ] Each commit in this PR is suitable for backport to the current stable branch