Mushroom Cards - Build a beautiful dashboard easily 🍄 (Part 1)

Does anyone know how to define a theme for mushroom that only has dark mode?

I have the default Mushroom rounded theme, and would like to toggle between light and dark mode using an automation so need to make a copy of the theme which only has ‘dark’ colours enabled - however checking inside the theme hardly anything is defined, as it’s only handling the rounded corners.
Is there any way to tell it to map all the ‘dark’ colours into the ‘light’ section of the theme? - I can’t actually seem to find the default theme file where they are set…

This is what is inside the Mushroom theme

Mushroom:
    # Home Assistant override
    ha-card-box-shadow: 0px 2px 4px 0px rgba(0,0,0,0.16)
    ha-card-border-radius: 12px
    modes:
        light:
          []     <----- Somehow I need to get all the 'dark' colours in here
        dark:
          []