Adding the boots would definitely make it more casual. You could also add a little khaki or jean jacket over top. That would help make the look more casual. Also, be careful about the jewelry you add. Make sure it isn't too dressy.
That dress is soo cute! lol and I love forever21! haha
anyway... I'm not sure how those kind of boots would look with that dress, but than again I havn't seen the dress, sooo yeah. Also, I think that maybe if you just wore like silver or grey ballet flats it would look fine. Good luck and hope this helped! :]