Provide dark app icon for dark themes

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## Feature request

**Which Nextcloud Version are you currently using:** v4.0.1

**Is your feature request related to a problem? Please describe.**
Since macOS Tahoe, switching to dark theme also enables dark app icons. Nextcloud only provides a light icon so far, which looks inconsistent and out of place:

I'm guessing this is also true for other platforms that allow switching between light and dark themes.

**Describe the solution you'd like**
Nextcloud should provide both a light and a dark app icon.