In Bootstrap 5 prior 5.3 there is an issue that the css variables for breakpoints are missing, which was fixed (by me) with version 5.3. Up to version 5.3, you can use the bootstrap-detect-breakpoint ...

An ES6 native replacement for select using standards Bootstrap 5 (and 4) styles. (almost) No additional CSS needed! Supports creation of new tags. Use attribute data-allow-new to allow creation of new ...