Globally altering styles

Question

Is there a generally applicable technique which will, for any tag/environment used in a LaTeX document, allow the default styling of the contents of instances of that tag/environment to be tweaked in the preamble?

• For instance, if I decided that all instances of \emph{} in the document should have their contents rendered not only italicised (a common default) but also bold, then what would I do?

• Similarly, if I decided that all instances of \begin{verbatim}...\end{verbatim} should have their contents rendered not only in a monospaced font (a common default) but also on a grey background, then what would I do?

But more importantly than simply explaining how to handle these two example cases, please can you outline a procedure a LaTeX user can follow in all cases in order to be able to change the styling of any given tag/environment.

Answer 1

Let's tackle only the first example. LaTeX declares \emph with

\DeclareTextFontCommand{\emph}{\em}

so we need to look up what \em does:

\DeclareRobustCommand\em{%
  \@nomath\em
  \ifdim \fontdimen\@ne\font >\z@
    \upshape
  \else
    \itshape
  \fi}

The first instruction, \@nomath\em raises an error if the command is found in math mode, otherwise it does nothing. The conditional checks whether the current font is slanted (in this case \fontdimen1\font is positive); if so it issues \upshape, otherwise \itshape.

So, if you want \emph to choose "boldface italic" in an upright context and "upright boldface" in an italic context you have to say

\DeclareRobustCommand\em{%
  \@nomath\em
  \ifdim \fontdimen\@ne\font >\z@
    \upshape\bfseries
  \else
    \itshape\bfseries
  \fi}

Is this a general method? No.

I won't even think to the changes necessary to print verbatim material on a grey background: the fancyvrb package provides such a feature and its code is very complicated.

In LaTeX you can't simply hand calls to the browser like HTML does. The two models are completely different.

Answer 2

As egreg says there is no general way -- each one needs to be considered separately. So for the \emph you could redefine the command in the preamble:

Note:

• As egreg pointed out, this required the use of \LetLtxMacro macro from the letltxmacro package instead of \let since \emph is declared with \DeclareRobustCommand.
• As Caramdir pointed out this may or may not be what is desired for nested use \emph{}. This redefinition yields an alternating bold italic, with bold upright, where as the original definition yields an alternating non-bold italic, with a non-bold upright as shown above.

Code:

\documentclass{article}
\usepackage{letltxmacro}

\LetLtxMacro\OldEmph\emph
\renewcommand{\emph}[1]{\textbf{\OldEmph{#1}}}%

\begin{document}
    \OldEmph{orignal emph}\par
    \emph{modified emph}\par
    \emph{modified emph with a \emph{nested emph}}
\end{document}