Use the dark svg for widget icon

[akhil1508]

📝 Summary

• Resolves: Dashboard widget title icon is black when it should be white #1475

🖼️ Screenshots

🏚️ Before	🏡 After

🚧 TODO

• ...

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• Documentation (README or documentation) has been updated or is not required

[max-nextcloud]

Thanks @akhil1508 for looking into this!

[max-nextcloud]

Looks good to me - in particular as that also seems to be what other apps are doing.

[github-actions]

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