With this, we can fix brave theme related bugs with multi profile because all profiles use browser-wide prefs. So far, system theme could be changed whenever new profile is used because brave theme was profile prefs but system theme is browser wide setting. And did cleanup and refactoring. Refactoring point is separation of theme managing and dark mode managing. So far, theme service has all dark mode handling logic. But I think theme service handles browser theme with underrlying dark mode. So, all dark mode logic is in dark_mode namespace. fix https://github.com/brave/brave-browser/issues/5373 fix https://github.com/brave/brave-browser/issues/5557