Create a brand new VCL application and add the Vcl.Themes and Vcl.Styles units to the main implementation form. These units are required to use VCL styles at runtime.

Drop on the form a TListBox component, two TButton components, and two TOpenDialog components. Leave the default component names.

Go to Project | Appearance and select eight styles of your choice from the list. Leave the Default style option to Windows.

The TStyleManager.StyleNames property contains all names of the available styles. In the FormCreate event handler, we have to load the already linked styles present in the executable to the listbox to let the user choose one of them. So, create a new procedure called StylesListRefresh with the following code and call it from the FormCreate event handler:

procedure TMainForm.StylesListRefresh;
var
  stylename: string;
begin
  ListBox1.Clear;
  // retrieve all the styles linked in the executable
  for stylename in TStyleManager.StyleNames do
  begin
    ListBox1.Items.Add(stylename);
  end;
end;

In the Button1Click event handler, we've to set the current style according to the one selected from ListBox1 using the following code:

  TStyleManager.SetStyle(ListBox1.Items[ListBox1.ItemIndex]);

The Button2Click event handler should allow the user to select a style from disk. So, we have to create a folder named styles at level of our executable and copy a few .vsf files from the default style directory which is C:\Program Files (x86)\Embarcadero\Studio\14.0\Redist\styles\vcl\ in RAD Studio XE6.

After copying the files, write the following code under the Button2Click event handler. This code allows the user to chose a style file directly from the disk. Then you can select one of the loaded styles from the listbox and click on Button1 to apply it to the application. The code is as follows:

if OpenDialog1.Execute then
begin
  if TStyleManager.IsValidStyle(OpenDialog1.FileName) then
  begin
    //load the style file
    TStyleManager.LoadFromFile(OpenDialog1.FileName);
    //refresh the list with the currently available styles
    StylesListRefresh; 
    ShowMessage('New VCL Style has been loaded');
  end
  else
    ShowMessage('The file is not a valid VCL Style!');
  end;
end;

Just to have an idea of how the different controls appear with the selected style, drag-and-drop some controls to the right-hand side of the form. The following screenshot shows an application with some styles loaded, some at design time and some from the disk. Hit F9 (or go to Run | Run) and play with your application using and loading styles from the disk.

The Style Chooser form with a Turquoise Gray style loaded