The MVC scaffolding uses Dojo for the calendar and validation. Within your scaffolded project there are two stylesheets imported in main.jsp
@import url("${pageContext.request.contextPath}/resources/dojo/resources/dojo.css");
@import url("${pageContext.request.contextPath}/resources/dijit/themes/tundra/tundra.css");
Those style sheets are resolved by the Spring Resources servlet. You can find a copy of the stylesheets in the spring-js-2.0.7.RELEASE.jar.
You’ll find in tundra.css, that there is a Calendar.css being imported.
Between dojo.css, tundra.css and Calendar.css you should be able to find the styles necessary to customize the calendar. Since it’s CSS, you should be able to just add your styles such that they will override those styles.