✨Add dark mode logo option

[declan-haojin]

This PR adds the option to specify a dark mode logo in the params.toml file as mentioned in the discussion. If no dark mode logo is specified, the default logo is used for both modes. If a dark mode logo is specified, the default logo is used for light mode and the dark mode logo is used for dark mode.

Thanks for creating such an amazing theme!

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[github-actions]

This pull request has been automatically marked as stale because it has not had any recent activity.
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[jpanther]

Thanks for the PR and sorry it's taken me so long to review it. I'm happy to include this feature as it's one that has been requested a few times. I'll make a few documentation additions and then merge it in.