Embed the report designer in .NET Framework, load a template, and connect its editing and save actions.

Reading and setting the theme. The current appearance is available through StiUXTheme.Appearance, which accepts Auto, Light or Dark:
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.Appearance = StiThemeAppearance.Dark;

Both the viewer and the designer follow the selected theme, so you can match Stimulsoft to the look of the host application — including automatic light/dark switching with the Auto mode.