I'm following the documentation and using everything at their default settings, but the text inside buttons is not centered vertically. I figured out if I set the line-height of .btn to 2.9rem (29px), then the text is centered. I think the line-height is set too high by default.

On your documentation website, the buttons have centered text, but if you open dev tools and uncheck the line-height for .btn, a border or outline or something shows up around the buttons, then if you recheck the line-height, the border is still there but now the text is aligned too low, just like the problem I'm having, so there is some issue with the line-height and getting the text centered in the buttons.