I believe that the problem you are having with the button disappearing is a typo in your script.js file.
You are switching out the stylesheet files on toggle. You're changing it to light_theme.css in the script, but your actual file is light-theme.css
@jesbalchiero
Posted
@AlexKMarshall OMG, I reviewed all the code and couldn't figure it out hahaha, thanks so much for the help!