Three themes added to all custom forms and controls:
Light
Dark Gray
Black
Theme selected automatically based on Office (2016 and later) and Windows settings.
If you change your Office Theme with the Advanced Spacing Pane open, it remains in the old theme. It can be redrawn in the new theme by activating it again from the Make Same menu.
The Settings page uses a built-in tab control with limited support for applying themes.
Message boxes use a system control and are always rendered in the light theme.
