Add a boxed text with the same theme in beamer

Question

I want to add a slide with only a subtitle which is inside a box of the same style as the current beamer style. Here is a picture of what I want. The second page is what I want (I Photoshop it, of course).

Thank you.

\documentclass[12pt]{beamer}

\mode<presentation>
\usetheme{Warsaw}
\usefonttheme{serif}
\setbeamertemplate{navigation symbols}{}
\beamertemplatenavigationsymbolsempty
\setbeamertemplate{footline}[page number]{}

\title{This is the main title}
\author{Patrick Li}
\institute{StackExchange.com}
\date{\today}

\begin{document}

\begin{frame}
\titlepage
\end{frame}

% Here should be the code to produce the second page in the above picture.

\end{document}

Answer 1

There are three possible answers:

1. by using \partpage
2. by using \titlepage
3. by mimicking the theme's template.

Using \partpage

If your aim is to generate a frame announcing the part's title you can use the \partpage command. To install such a command every time a part is created just write in your preamble

\AtBeginPart{\frame{\partpage}}

The contents of the part page are controlled by the part page template that you can modify with \setbeamertemplate{part page}{<contents>}.

Using \titlepage

If you want to generate a title page with different title, subtitle etc just set the new values and call \titlepage

\begin{frame}
    \title{Part I}\author{}\date{}\institute{}\subtitle{}
    \titlepage
\end{frame}

Even better, if you are using this to print the part's title you can use

\title{\insertpart}

and if you want beamer to generate such frame at each part you can pass the commands to \AtBeginPart in your preamble as in

\AtBeginPart{
    \begin{frame}
        \title{\insertpart}\author{}\date{}\institute{}\subtitle{}
        \titlepage
    \end{frame}
}

Mimicking the theme

Suppose you are using the default theme. The definition of the title page element is in beamerinnerthemedefault.sty and contains the following snippet to generate the blue box:

\begin{centering}
  \begin{beamercolorbox}[sep=8pt,center,#1]{title}
    \usebeamerfont{title}\inserttitle\par%
    \ifx\insertsubtitle\@empty%
    \else%
      \vskip0.25em%
      {\usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\insertsubtitle\par}%
    \fi%     
  \end{beamercolorbox}%
  ...
\end{centering}

so you can just put a beamercolorbox in your frame as in

\begin{beamercolorbox}[sep=8pt,center]{title}
    Part I
\end{beamercolorbox}

However you will need to change the parameters to match your theme's design.