One syntax error in your css means that RMSDK will ignore it all. Browser-based renderers like Sigil are more forgiving.
Validate your stylesheet(s). Sigil makes it easy to do with Tools-->Validate Stylesheets with W3C.
Code:
.h4 {
font-size: 1.2em;
margin-top: 1em;
margin-bottom 1em;
text-align: center
}
Colon missing after margin-bottom.