ShellDefaultPageExtension.cs

public class ShellDefaultPageExtension : Sitecore.Shell.DefaultPage
{
	protected override void OnInit(EventArgs e)
	{
		base.OnInit(e);

		var control = this.Controls[0];

		var page = control.Page;

		if (page.ClientQueryString.Equals("xmlcontrol=Installer.InstallPackage"))
		{
			page.Header.Controls.Add(new LiteralControl("<script type='text/JavaScript' src='/sitecore/shell/Controls/Lib/jQuery/jquery-1.12.4.min.js'></script>"));
			page.Header.Controls.Add(new LiteralControl("<script type='text/javascript' language='javascript' src='/sitecore/toastr/toastr.js'></script>"));
			page.Header.Controls.Add(new LiteralControl("<link href='/sitecore/toastr/toastr.css' rel='stylesheet'/>"));
		}
	}
}