YESSSS!! I've been many days thinking about how could I manage to achieve something like this, setting my two custom themes instead of using the default dynamic theme which I don't really like, and finally found it! Thanks a lot Simon for your idea and your time invested on this!!

btw, the themes I use and I totally love are:
Day: "Google Chrome Light" https://addons.mozilla.org/es/firefox/addon/google-chrome-light/
Night: "Google Chrome Dark" https://addons.mozilla.org/es/firefox/addon/google-chrome-dark/

Both made by the same user, StyleFox