The quiz navigation buttons in the side bar will become invisible if you change the btn-secondary color to be white (which would generally be a side effect of turning the button color itself darker to the point that dark text no longer works on it).
This is because the buttons declare themselves as being .btn-secondary and so inherit the colors from that, but then override only the background to be $card-bg, without changing the text color.
If $card-bg and btn-secondary text color are similar, the buttons become unreadable. if they're identical then no-one will be able to read them, but if they're just close then it'll be hard for people with vision impairment to read them.
I think the general solution is not to use btn-secondary on buttons that are intended to have a visual display different from btn-secondary, so possibly some special btn- class for these buttons, as they are quite unique in their behaviour and appearance. Possibly this should use color-yiq to figure out if the text should be dark or light given the buton background color.