Hi,
I just finished this challenge recently myself. I think you did a stellar job! The only few things I would look at would be:
the drop shadow around the main card. I would lighten the rgb value.
I think the opacity is good. And maybe add a little more spread.
the sign up button I think needs a hover state, as per the README.md
the last is the media query. When it gets down to the 1000px browser width, and it
displays the mobile version, the card actually expands to larger then the desktop
card version and the signup button spreads across the whole width. You probably
want a smoother transition between the two versions.