themeAnimationDuration property
final
The duration of animated theme changes.
When the theme changes (either by the theme, darkTheme or themeMode parameters changing) it is animated to the new theme over time. The themeAnimationDuration determines how long this animation takes.
To have the theme change immediately, you can set this to Duration.zero.
The default is kThemeAnimationDuration.
See also: themeAnimationCurve, which defines the curve used for the animation.
Implementation
final Duration themeAnimationDuration;