Starting from version 2.1.0 rmdshower is able to use custom shower themes, defined in separate R packages.
The name of the theme package must have an rmdshower. prefix (note the dot!), and it must be installed on the system.
An example theme package is at https://github.com/MangoTheCat/rmdshower.mango, this contains the Mango theme. I suggest that you use this package as a template. Most files of the package are generic, and need no modifications.
The /node directory is the source code of theme, and /inst/package is the installed theme. /inst/package is generated automatically by node.js, and should not be modified directly.
The style files are written in SCSS, and are in /node/styles. Other assets are in other directories within /node.
To generate /inst/package from /node, call the update.sh script. It needs to have node.js installed, and it installs the required npm packages automatically, these are specified in /node/package.json. Ideally you would modify package.json to define a new npm package, but this is not strictly necessary.
A /inst/style-override.css file can be used to override the generated CSS styles. This file is included last in the final HTML document.
The /inst directory might also contain an example Rmd file, and the other assets needed for it.
To use a theme package in an Rmd file, you can simply refer to its name in the theme parameter of the YAML header, and rmdshower will look up the package and insert the appropriate CSS and other files in the output:
---
title: "Shower Presentations with R Markdown"
author: "Gábor Csárdi"
output:
rmdshower::shower_presentation:
theme: mango
---