Use the adblocker and the :style() rule
You can read about it here https://developer.chrome.com/blog/auto-dark-theme/#per-element-opt-out how Chrome developers talk about what to do.
example.com##:root:style(color-scheme: only light !important;) and done, that will turn it off in whatever domain you want completely.
it can also be used per element, but that can only be done through Desktop devtools easily, same process, just instead of :root you use a selector and maybe modify other properties like background to make it compatible, or remove an incompatible property that Auto dark mode can’t ‘auto dark’'. :style() makes all possible.
You add custom Adblocker rules in Brave://Adblock