How do you adjust the transparency/opacity of active and non-active sections in the beamer footer?

Question

I'm using pdflatex, beamer class, boxes theme.

I have a footline with the sections of my document listed there. It slightly grays out the non-active section titles in the footer, but not enough.

How do I alter the transparency of non-active section headings in the footer?

\setbeamertemplate{footline} {
  \leavevmode%
  \hbox{%
  \pgfsetfillopacity{95}
  \begin{beamercolorbox}[wd=1.0\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
    \usebeamerfont{author in head/foot}%
    \insertsectionnavigationhorizontal{.8\paperwidth}{}{}%
  \end{beamercolorbox}
  }
}