No space before or after the theorem environment

Question

I am sorry if this have been asked before, but I tried searching for similar questions and none of them answered my issue.

I defined a new theorem environment style, which I called "slanted." However, despite using \topsep, the new theorem style is not separated by any space from above or below. Does anyone know why that could be the case? Is it possible that my preamble is defined incorrectly? Below you can find the preamble that I am using together with the produced output (for comparison, I also included the definition and remark environments). Any help would be greatly appreciated.

EDIT 1: This issue has to have something to do with me using amsart. But why? Why can't I properly define new theorem styles in amsart? I am fairly new to amsart, so maybe that is the case. Do I even need to load amsmath, amssymb and amsthm or are they loaded automatically when I use amsart?

\documentclass[12pt]{amsart}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{times,newtxmath}

\newtheoremstyle{slanted}
  {\topsep}%   Space above
  {\topsep}%   Space below
  {\slshape}%  Body font
  {}%          Indent amount (empty = no indent, \parindent = para indent)
  {\bfseries}% Thm head font
  {.}%         Punctuation after thm head
  {0.5em}%     Space after thm head: " " = normal interword space;
     %         \newline = linebreak
  {}%          Thm head spec (can be left empty, meaning `normal')

\theoremstyle{slanted}
\newtheorem{theorem}{Theorem}

\theoremstyle{definition}
\newtheorem*{de}{Definition}

\theoremstyle{remark}
\newtheorem*{rem}{Remark}

\begin{document}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{theorem}
Mathematicians seek out patterns.
\end{theorem}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{de}
Mathematicians seek out patterns.
\end{de}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\begin{rem}
Mathematicians seek out patterns.
\end{rem}

Mathematicians resolve the truth or falsity of conjectures by mathematical proof.
\end{document}

Answer 1

You have a \topsep of roughly 2.5pt plus 2.5pt, that is hardly noticed. With some like \setlength{\topsep}{5em} before of the new theorem style definition, you will see a big space before and after the theorem.

But instead of change a length that could be used elsewhere, it could be better use a new length for this, or just some like:

\newtheoremstyle{slanted}
  {1em plus .2em minus .1em}%   Space above
  {1em plus .2em minus .1em}%   Space below
 ....