I have replied in that “github discussion” thread you have linked there, but to summarize: I suspect you have your operating system / browser’s dark theme turned on. Simply disable this on your computer/browser if you don’t want modern websites to reflect your dark color scheme preference.
Automatic dark mode switching is a very desired feature for me (and I would want that for kanboard itself, too). I can’t use a kanban board or read documentation at night if it’s black-on-white instead of white-on-black, it burns my eyes.