This is not new but has "always" been like that.

From te skinning documentation:

DotNetNuke uses an external style sheet ( or CSS ) specification which takes full
advantage of their cascading nature. Essentially this means that DotNetNuke has
multiple external style sheet references on a page – each style sheet reference is
specified in prioritized order so that hierarchical overriding can occur. The
cascading order of style sheets is summarized below ( with each item overriding the
previous items ):
o Modules – styles for custom modules defined in
PortalModuleControl.StyleSheet
o Default – default host level styles – default.css
o Skin – skin styles – skin.css or skinfilename.css
o Container – container styles – container.css or containerfilename.css
o Portal – custom styles defined by portal Administrator – portal.css