Minor revision to SASS files

* Add another variable for repeating value of `1.5em` as spacing for easier maintenance
* Removing `display: inline` when using `float` since the `display` will always be set to `block` anyway. [(Ref)](http://www.w3.org/TR/CSS2/visuren.html#dis-pos-flo)
* Add mixin for `appearance` since Firefox still need a prefix.