The Window themeColor configuration does not change the appearance of the component.
The Window colors are not changed.
The component appearance should be changed based on the configured value in themeColor.
<style>
.k-window-titlebar {
color: var(--kendo-color-on-dark, #ffffff);
background-color: var(--kendo-color-dark, #3d3d3d);
}
</style>
Or
$('.k-window').addClass('k-window-dark')