theme.park/docker-mods/plex/root/etc/cont-init.d/98-themepark

#!/usr/bin/with-contenv bash

echo '-------------------------'
echo '|  Plex theme.park Mod  |'
echo '-------------------------'

# Display variables for troubleshooting 
echo -e "Variables set:\\n\
'TP_DOMAIN'=${TP_DOMAIN}\\n\
'TP_THEME'=${TP_THEME}\\n"

APP_FILEPATH=''
if [ "${TP_HOTIO}" = true ]; then
    echo 'Changing to Hotio file path!'
    APP_FILEPATH='/app'
fi

# Set default
if [[ -z ${TP_DOMAIN} ]]; then
    echo 'No domain set, defaulting to theme-park.dev'
    TP_DOMAIN='theme-park.dev'
fi

case ${TP_DOMAIN} in
  *"github.io"*)
  echo "Switching to github.io URL style"
    TP_DOMAIN="${TP_DOMAIN}\/theme.park"
    ;;
esac

if [[ -z ${TP_THEME} ]]; then
    echo 'No theme set, defaulting to organizr-dark'
    TP_THEME='organizr-dark'
fi

# Adding stylesheets     
if ! grep -q "${TP_DOMAIN}" "${APP_FILEPATH}"; then
    echo '---------------------------'
    echo '|  Adding the stylesheet  |'
    echo '---------------------------'
    sed -i "s/<\/head>/<link rel='stylesheet' href='https:\/\/${TP_DOMAIN}\/CSS\/themes\/plex\/plex-base.css'><\/head> /g" "${APP_FILEPATH}"/usr/lib/plexmediaserver/Resources/Plug-ins-*/WebClient.bundle/Contents/Resources/index.html
    sed -i "s/<\/head>/<link rel='stylesheet' href='https:\/\/${TP_DOMAIN}\/CSS\/variables\/${TP_THEME}.css'><\/head> /g" "${APP_FILEPATH}"/usr/lib/plexmediaserver/Resources/Plug-ins-*/WebClient.bundle/Contents/Resources/index.html
    printf 'Stylesheet set to %s\n' "${TP_THEME}"
    if [[ -n ${TP_ADDON} ]]; then
        for addon in $(echo "$TP_ADDON" | tr "|" " "); do
        sed -i "s/<\/head>/<link rel='stylesheet' href='https:\/\/${TP_DOMAIN}\/CSS\/addons\/plex\/${addon}\/${addon}.css'><\/head> /g" "${APP_FILEPATH}"/usr/lib/plexmediaserver/Resources/Plug-ins-*/WebClient.bundle/Contents/Resources/index.html
        printf 'Added custom addon: %s\n\n' "${addon}"
        done
    fi
fi