Halfmoon is a highly customizable, drop-in Bootstrap replacement. It comes with three built-in core themes, with dark mode support for all themes and components.
I will do a better investigation/write up later but just to put this on your radar for now...
Reduced test case on Codepen showing navbar bottom dropdown bug on small screens and down (width <= 768px). Dropdown appears briefly and then is hidden.
Nevermind now that I have read the Dropdown docs I see now I can add the dropup class to the dropdown. Updated the test case on Codepen and closing this.
I will do a better investigation/write up later but just to put this on your radar for now...