Steps to reproduce:

Create a new project
Open Edit > Project Settings > UI Toolkit and enable the "Enable Multiple UI Builder Windows" toggle
In the Project window, right-click > Create > UI Toolkit > UI Document; double-click the UXML to open it in UI Builder
Open a second UI Builder window: Window > UI Toolkit > UI Builder (New Window), and open the SAME UXML in it
In the Canvas toolbar of the FIRST window, change the Preview Theme via the Theme dropdown to a different theme
Observe the second window's canvas and both windows' Theme dropdown area
In the SECOND window (where the theme did NOT change), click the "Reset Preview Theme" button that appeared next to its Theme dropdown

Actual results:

Step 6-7: The theme change is applied only in the first window's canvas; the second window's canvas keeps the old theme (the preview theme is not synced between the two windows), but the "Reset Preview Theme" button appears in BOTH windows.
Step 8: Clicking "Reset Preview Theme" in the second (unchanged) window removes the button from BOTH windows, but does not revert the previewed theme in either window (the first window keeps the changed theme; nothing is reset).

Expected results: The two windows share a consistent preview-theme state: either the theme change is reflected in both, or the "Reset Preview Theme" button only appears in the window that actually has an unsaved theme change, and clicking Reset reverts that window's preview theme to the project-wide Preview Theme (Project Settings > UI Toolkit > Project Preview Theme). The Reset control must actually reset.

Reproducible with versions: 6000.7.0a6

Can’t test with versions: 6000.0.84f1, 6000.3.24f1, 6000.5.11f1, 6000.6.1f1 (No multiple UI Builder Windows feature)

Tested on (OS): Windows 11