The WPF viewer and designer follow a global UI theme — an appearance (Auto/Light/Dark) and an accent color — that you can read and change at run time.

Read and apply the theme. The current values come from StiUXTheme; ApplyNewTheme switches them live:
// Read the current appearance
switch (StiUXTheme.Appearance)
{
    case StiThemeAppearance.Auto:  comboBoxAppearance.SelectedIndex = 0; break;
    case StiThemeAppearance.Light: comboBoxAppearance.SelectedIndex = 1; break;
    case StiThemeAppearance.Dark:  comboBoxAppearance.SelectedIndex = 2; break;
}

// Apply a new appearance and accent color
StiUXTheme.ApplyNewTheme(StiThemeAppearance.Dark, StiThemeAccentColor.Blue);

How it works. Both the viewer and the designer subscribe to the global theme, so one call restyles the entire Stimulsoft UI to match your application.