Steps to reproduce:
1. Create new Unity project
2. Open UI Builder window (Window > UI Toolkit > UI Builder)
3. In Library enable Editor Extension Authoring
4. In Hierarchy add "ToolbarButton" and "Toolbar" elements
5. Below Viewport, change from "Active Editor Theme (Project)" to "Light Editor Theme"
6. Observe Button and toolbar elements in preview Canvas window
Actual results: If Editor theme in General settings is set to Dark and changing active theme in UI Builder to Light, elements stay dark themed, elements don't adapt to switched theme in preview window
Expected results: Elements in preview window switch to Active theme (Light or Dark), doesn't matter Editor theme
Reproducible with versions: 6000.0.64f1, 6000.2.15f1, 6000.3.2f1, 6000.4.0b2, 6000.5.0a3
Tested on (OS): Ubuntu 24.04, macOS Tahoe 26.1 (M1)