Invisible Assitant icon in Toolbar

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Short Description: [One-line issue summary for stakeholders]
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Problem/Motivation

The icon of the Assistant button in the Toolbar is invisible on a basic installation using Claro as the admin theme.

Steps to reproduce (required for bugs, but not feature requests)

Please provide information like AI modules enabled, which AI provider, browser, etc.

Configure an AI chat and set its position to "Toolbar". (Watch a tutorial on how to add an AI chat.)

Proposed resolution

Update the styling of the button.

Remaining tasks

Update the styling of the button.

Optional: Other details as applicable (e.g., User interface changes, API changes, Data model changes)

This bug appears in all versions.

AI usage (if applicable)

[ ] AI Assisted Issue
This issue was generated with AI assistance, but was reviewed and refined by the creator.

[ ] AI Assisted Code
This code was mainly generated by a human, with AI autocompleting or parts AI generated, but under full human supervision.

[ ] AI Generated Code
This code was mainly generated by an AI with human guidance, and reviewed, tested, and refined by a human.

[ ] Vibe Coded
This code was generated by an AI and has only been functionally tested.