4

Here is my minimal code:

\documentclass[12pt,letterpaper]{article}

\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{pgfornament}

\newtheorem{thm}{Theorem}

\newcommand*{\centeredornament}{\begin{center}\pgfornament{88}\end{center}}
\newenvironment{theorem}[1][]{\centeredornament\begin{thm}[#1]\upshape}{\end{thm}\centeredornament}

\begin{document}
 \begin{theorem}[This is a sentence. ]
  This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. 
 \end{theorem}

\end{document}

It produces the following as output:

enter image description here

The black portions mark the width of the page. See how the ornaments are shifted to the right, rather than actually being "centered" on the page width? How can I fix this?

6

You should add option [width=13cm] to your used command \pgfornament like

\pgfornament[width=13cm]{88}

Without this width the ornament is to long to fit into the textwidth of your text.

You can use

\usepackage{showframe}

to visualize the typing area and margins.

With the complete MWE

\documentclass[12pt,letterpaper]{article}

\usepackage{showframe} % <==============================================
\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{pgfornament}

\newtheorem{thm}{Theorem}

\newcommand*{\centeredornament}{\begin{center}\pgfornament[width=13cm]{88}\end{center}} % 88
\newenvironment{theorem}[1][]{\centeredornament\begin{thm}[#1]\upshape}{\end{thm}\centeredornament}

\begin{document}
\begin{theorem}[This is a sentence. ]
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. This is a sentence. This is a sentence. 
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. This is a sentence. This is a sentence. 
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. This is a sentence. This is a sentence. 
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. This is a sentence. This is a sentence. 
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. This is a sentence. This is a sentence. 
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. This is a sentence. This is a sentence. 
  This is a sentence. This is a sentence. This is a sentence. This is a 
  sentence. This is a sentence. 
\end{theorem}

\end{document}

you get this result:

enter image description here

Without [width=13cm] you get the following result (see the ornament line reaching the margin):

enter image description here

5

Your ornaments are actually stretching beyond the margins, as the overfull vbox warnings your code issues, tell you.

enter image description here

So, you could set things up better with a \makebox also taking care of indent with:

\newcommand*{\centeredornament}{\noindent\makebox[\textwidth][c]{\pgfornament[width=1.1\textwidth]{88}}}

The [width=1.1\textwidth] for the ornament is there to deliberately have it a little larger than textwidth (but centered), as it seems to be your intent. But you can set this value as you please, of course.

In full:

\documentclass[12pt,letterpaper]{article}

\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{pgfornament}
\usepackage{showframe}

\newtheorem{thm}{Theorem}

\newcommand*{\centeredornament}{\noindent\makebox[\textwidth][c]{\pgfornament[width=1.1\textwidth]{88}}}

\newenvironment{theorem}[1][]{\centeredornament\begin{thm}[#1]\upshape}{\end{thm}\vspace{-1ex}\centeredornament}

\begin{document}

 \begin{theorem}[This is a sentence. ]
  This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence. This is a sentence.
 \end{theorem}

\end{document}

enter image description here

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