Burger button icon in the Image Reference is not light theme friendly when generating a new 3D Object

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Open the Package Manager window
3. Install the AI Assistant package (com.unity.ai.assistant)
4. Generate a new 3D Object (GameObject > 3D Object > Generate 3D Object)
5. Observe the burger button icon in the Image Reference field

Actual results: The burger button icon is not light theme friendly

Expected results: The burger button icon is light theme friendly

Reproducible with versions: 1.6.0-pre.1, 2.1.0-pre.1 (6000.0.70f1, 6000.3.11f1, 6000.4.0b12, 6000.5.0a8)

Tested on (OS): MacOS Silicon Tahoe 26.2