Low Contrast/Poor Visibility for Selected Items in App UI Icon Browser (Light Theme)

Steps to reproduce:

1. Create a new Unity project 
2. Set Unity Theme to "Light" 
3. Install the App UI package (com.unity.dt.app-ui)
4. Open the Icon Browser window (Window > App UI > Icon Browser)
5. Create a new App UI Icons style sheet
6. Click the "Show required icons" toggle to populate the grid interface
7. Left-click on any icon tile within the grid view matrix
8. Look closely at the borders and background state of the targeted tile

Actual results: The selection overlay is incredibly faint. The bounding indicator blends directly into the adjacent grid lanes

Expected results: Selected items must feature a high-contrast, distinct visual state such as a bold theme accent border (Unity blue) or a clear background color overlay ensuring instant accessibility regardless of whether Light or Dark mode is selected.

Reproducible with versions: 2.1.10 (6000.0.76f1, 6000.3.17f1, 6000.4.10f1, 6000.5.0b11, 6000.6.0a6)

Tested on (OS): Tahoe 26.5

Notes: Unity Standards - US-0171