If you apply one theme to the entire application and want to apply another theme for RadDock as it is demonstrated in this help article, the ThemeName property of RadDock is not respected:
public RadForm1()
{
InitializeComponent();
radDock1.ElementTree.EnableApplicationThemeName = false;
radDock1.ThemeName = "FluentDark";
ThemeResolutionService.ApplicationThemeName = "Windows7";
}