How to set light mode as default theme? (even if I have a custom theme)

What is the default Streamlit theme when there is no custom theme? System theme mode or light?

If the Streamlit default theme is ‘Use system setting’, how do we explicitly set the default theme as ‘light’? (instead of having to explicitly define the full light theme like this as a custom theme)

Basically, I do not want the custom theme to be the default theme.