[Tex/LaTex] How to use the color of an alert block for a ruler

beamerblockcolorpositioningvertical alignment

I have a beamer presentation and an alert block. I want to insert a horizontal ruler to seperate text. To fit into the style of the presentation, it should match the color of the surrounding alert block. How can I use the color of the alert blocks title line? At the moment I am just using a color that matched okay-ish in my opinion.

\begin{center}
    \textcolor{BrickRed}{\hrulefill}
\end{center}

I am aware of the command \usebeamercolor[fg]{structure} text, but I do not know what to put for fg and structure to give be the desired color (yes, I checked the beamer documentation).

enter image description here

Extra: At the moment, the exact vertical positioning of the \hrulefill is achieved by two (annoyingly fine-tuned) \vspace-commands, one before and one after the ruler. Without these, there are huge gaps between the text and the ruler. Is there a better way to achieve a nice positioning?

Update

The problem with the color seems to be solved by the answer of samcarter. For the positioning problem I was pointed out to provide an MWE:

\documentclass[pdf]{beamer}

\usetheme{Malmoe}
\usecolortheme{beaver}
\usecolortheme{orchid}
\usefonttheme{professionalfonts}

\begin{document}

\begin{frame}{Frenet equations}

\begin{alertblock}{\textbf{Theorem:} Frenet equations}
    For the Frenet frame $(\bf t,\bf n)$ of a curve holds
    \begin{align*}
        \bf t'(t) &= \phantom+\kappa(t)\bf n(t),\\
        \bf n'(t) &= -\kappa(t)\bf t(t).
    \end{align*}
    %
    \vspace{-3em}
    \begin{center}
        \usebeamercolor[bg]{block title alerted}\hrulefill
    \end{center}
    %
    \vspace{-0.5em}
    The equations can be written in \emph{matrix form}:
    $$
        \begin{pmatrix}
        \bf t'\\\bf n'
        \end{pmatrix}=\begin{pmatrix}
        \phantom-0&\kappa\\-\kappa&0
        \end{pmatrix}\begin{pmatrix}
        \bf t\\\bf n
        \end{pmatrix}.
    $$
\end{alertblock}

\end{frame}

\end{document}

Best Answer

You can access the colour with \usebeamercolor[bg]{block title alerted}

\documentclass{beamer}

\usecolortheme{orchid}

\begin{document}

\begin{frame}
    \begin{alertblock}{block title}
        content...
    \end{alertblock}

    {\usebeamercolor[bg]{block title alerted}\rule{\textwidth}{2pt}}

\end{frame} 

\end{document}

The large vertical spacing mainly comes from the center environment, which is not necessary anyway for an element which spans the whole line. 

\documentclass[pdf]{beamer}

\usetheme{Malmoe}  %% Themenwahl
\usecolortheme{beaver}
\usecolortheme{orchid}
\usefonttheme{professionalfonts}

\begin{document}

\begin{frame}{Frenet equations}

\begin{alertblock}{\textbf{Theorem:} Frenet equations}
    For the Frenet frame $(\bf t,\bf n)$ of a curve holds
    \begin{align*}
        \bf t'(t) &= \phantom+\kappa(t)\bf n(t),\\
        \bf n'(t) &= -\kappa(t)\bf t(t).
    \end{align*}%

    \vspace{-1em}
    {\usebeamercolor[bg]{block title alerted}\hrulefill}

    The equations can be written in \emph{matrix form}:
    \[\begin{pmatrix}
    \bf t'\\\bf n'
    \end{pmatrix}=\begin{pmatrix}
    \phantom-0&\kappa\\-\kappa&0
    \end{pmatrix}\begin{pmatrix}
    \bf t\\\bf n
    \end{pmatrix}.\]
\end{alertblock}

\end{frame}

\end{document}
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