Though if there was one thing, the form submit buttons feels… off. It has a different font-family as well as different font-size compared to the rest of the page. I’d suggest making it match, just for consistency’s sake. Adding a simple font-family: inherit; to your .btn class takes care of that.
It’s just one of the weird quirks of input type buttons that even though a parent has a set value, it just gets ignored for… reasons.