% !TeX root = test.tex
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% The rainbow colour theme 
% A beamer colour theme which alternates theme colours on every frame
% Maintained by samcarter
%
% Project repository and bug tracker:
% https://github.com/samcarter/beamertheme-rainbow
%
% Released under the LaTeX Project Public License v1.3c or later
% See http://www.latex-project.org/lppl.txt
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\ProvidesPackage{beamercolorthemerainbow}[2023/07/04 version v0.1 Beamer colour theme with alternating theme colours]

\mode<presentation>

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%
% Default colours
%
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\definecolor{rainbow@0}{HTML}{e50000}
\definecolor{rainbow@1}{HTML}{ff8d00}
\definecolor{rainbow@2}{HTML}{ffee00}
\definecolor{rainbow@3}{HTML}{028121}
\definecolor{rainbow@4}{HTML}{004cff}
\definecolor{rainbow@5}{HTML}{770088}
\colorlet{beamer@structure@color}{rainbow@0}
\setbeamercolor{structure}{fg=beamer@structure@color}

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%
% Reading theme options
%
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\DeclareOptionBeamer{colors}{\def\rainbow@colors{#1}}
\ExecuteOptionsBeamer{colors={rainbow@0,rainbow@1,rainbow@2,rainbow@3,rainbow@4,rainbow@5}}
\ProcessOptionsBeamer

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%
% Defining colours
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\ExplSyntaxOn

% list to store colour list
\clist_new:N \l_rainbow_clist
\clist_set:Nx \l_rainbow_clist \rainbow@colors

% setting colour based on frame number
\pretocmd{\beamer@@@frame}{
  \colorlet{beamer@structure@color}{
    \clist_item:Nn \l_rainbow_clist {
      \int_eval:n {
        \int_mod:nn { \insertframenumber }{ 
          \clist_count:N \l_rainbow_clist
        } + 1
      }
    }
  }
}{}{}

\ExplSyntaxOff

\mode
<all>