How to do it...

Style manipulation at runtime is done using the class methods of the TStyleManager class. Follow these steps to change the style of your VCL application at runtime:

1. Create a brand new VCL application and add the Vcl.Themes and Vcl.Styles units to the main form's implementation uses section. These units are required to use VCL styles at runtime.
2. Drop on the form a TListBox, two TButton, and a TOpenDialog. Leave the default component names.
3. Go to Project | Appearance and select eight styles of your choice from the list. Leave the Default style as Windows.
4. The TStyleManager.StyleNames property contains the names of all 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; 

5. In the Button2Click event handler, we set the current style according to the one selected from the ListBox1 using the code that follows:

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

6. The Button1Click event handler should allow the user to select a style from the disk. So, we have to create a folder named styles at the level of our executable and copy a .vsf file from the default styles directory, which in RAD Studio 10.2 Tokyo is C:\Program Files (x86)\Embarcadero\Studio\19.0\Redist\styles\vcl.
7. After copying, write the following code under the Button1Click event handler. This code allows the user to choose 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 application:

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; 

8. Just to give you an idea of how the different controls appear with the selected style, drag and drop some controls onto the right-hand side of the form.

9. Hit F9 (or go to Run | Run), and play with your application, using and loading styles from the disk. The following screenshot shows my application with some styles loaded, some at design time and some from the disk: