Questions on macro writing in TeX to modify an existing style file: fancybox.sty

I'd do something like this to parameterise the macro

\documentclass{article}

\usepackage{fancybox}
\usepackage{keyval}


\makeatletter

\define@key{myfb}{inner}{\def\myfb@inner{#1}}
\define@key{myfb}{outer}{\def\myfb@outer{#1}}
\define@key{myfb}{sep}{\def\myfb@sep{#1}}
\newcommand\mydoublebox[1][]{%
\def\myfbkeys{\setkeys{myfb}{#1}}%
\VerbBox\@doublebox}
\def\@doublebox#1{%
  \begingroup
    \def\myfb@inner{.75\fboxrule}%
    \def\myfb@outer{2\fboxrule}%
    \def\myfb@sep{\fboxrule+.5pt}%
    \myfbkeys
    \setbox\@fancybox\hbox{{#1}}%
    \fboxrule\dimexpr\myfb@inner\relax
    \setbox\@fancybox\hbox{\fbox{\box\@fancybox}}%
    \fboxrule\dimexpr\myfb@outer\relax
    \fboxsep\dimexpr\myfb@sep\relax
    \fbox{\box\@fancybox}%
  \endgroup}
\makeatother

\begin{document}

\mydoublebox{hello}

\mydoublebox[inner=4pt,sep=10pt]{hello}

\end{document}

Also don't do this

{\huge A HISTORY OF\\
THE\\
MYERS\\
OVERSTREET\\
and\\
GRAY\\
FAMILIES\\}

As that ends the font size before paragraph ends so sets huge text on a normal baseline. (You do the same with \small and possibly some other size changes)

I am providing a more general answer to your questions:

How to a create a parametric version of \doublebox?

In the absence of any pre-defined hooks in an existing macro, you have three choices:

1. Re-write the existing macro fully.
2. Inject code via using the LaTeX2e macro \g@addto@macro or using similar macros from the etoolbox package.
3. Use the existing macro and add parameters, using a key-value interface.

I personally prefer a combination of 1) and 3), which I will explain in detail below using the LaTeX macro \rule as an example, which might come handy for your particular case.

The normal command has the format:

     \rule[<raised>]{<width>}{<height>}   

Personally, I have trouble remembering if the width comes first or the height when calling the macro, also it would be nice if one can set the color as well. A command of the form:

   \Rule[rule color = thegray,
            rule thickness = 1pt,
            rule raised = 2pt,
            rule width = 85pt]      

is preferable, as the key values can be typed in any order and also one can set default values at the beginning of a document. If you notice I capitalized the name of the macro as it is considered good practice to try and not change existing macros, if possible.

I also use PGF keys, as I find it quicker to code them.

\documentclass{article}
\usepackage{pgf}

\definecolor{thegray} {rgb}{0.9,0.9,0.9}
\def\setcolor#1{\color{#1}}

% create family of keys called rule
\pgfkeys{/rule/.is family}
\def\cxset{\pgfqkeys{/rule}}


\cxset{rule width/.store in = \rulewidth@my,
       rule thickness/.store in=\rulethickness@my,
       rule color/.code ={\setcolor{#1}},
       rule raised/.store in = \ruleraised@my
      }

\cxset{rule thickness = 10pt,
       rule raised = 2pt,
       rule width = 45pt}

\newcommand\Rule[1][rule color = thegray,
       rule thickness = 1pt,
       rule raised = 2pt,
       rule width = 85pt]{%
  \colorlet{originalcolor}{.}%
  \cxset{#1}%
  \begingroup
  \rule{\rulewidth@my}{\rulethickness@my}%
  \endgroup
  \color{originalcolor}}

\begin{document}
\Rule

\Rule[rule width=60pt, 
      rule color= purple]

test
\end{document}

See if you can use this approach and modify \doublebox to your requirements. If you succeed post the answer.