[Feature request]Theme change fading effect.

Is your feature request related to a problem? Please describe.
When the theme changes from light to dark there is a sudden increase in the brightness of the screen. The sudden increase from dark to light hurts my eye a little bit when I change the theme from dark to light especially when a major portion of the screen is set to brush RegionBrush or SecondaryRegionBrush. I don't know how many people are there whose eyes might also hurt when they see a sudden increase in brightness from dark to light.

Describe the solution you'd like
Instead of switching from dark to light all of a sudden we could increase the brightness gradually by fading the color from dark to light. That might not hurt the eyes.

So I'm suggesting, when the theme changes the theme colors should change gradually instead of suddenly.

Start by locating the theme-change handling in HandyControl's WPF code; the issue does not name a file, entry point, or test. Determine how theme colors involving RegionBrush and SecondaryRegionBrush are switched, then define and verify a gradual transition without the reported brightness jump.