It's difficult to have a unifying theme when your coalition is made up of multiple blocs you have to juggle because they aren't natural allies as much as circumstantial ones.
This actually applies in both parties.
This is actually really important. The "unifying theme" of a political party is generally just the one common issue that the disparate interest groups currently forming a coalition in that party can agree on.
The Republican unifying theme is "less federal government" because all the Republican interest groups -- Randians, libertarians, racists, and social reactionaries -- agree that the only way they'll get to put their goals into place is with less federal government. Part of the big reason the Republicans are so bad at governing is that, given actual control of federal power, it turns out that the positive goals of the Republican party coalition are not common at all -- they all want to do DIFFERENT insane things, and generally think the positive agendas of the other groups are at best unimportant and at worst actually a bad idea.
Insofar as there's a Democratic unifying theme today it's something like "less social restrictions" because all the Democratic interest groups -- women, GLBT, people of color, technocrats, Millenials, inexplicable rich white liberals -- agree that codified social restrictions are usually discriminatory. This is still a pretty effective positive message because intersectionality works well enough for everybody to agree that laws against gay marriage, abortions, driving while black, etc. are pretty clearly bad social restrictions. Eventually we will run out of low-hanging fruit, though, and it will be more difficult to explain how enacting a citizen's wage is fundamentally about repealing a social restriction created by nature, and then we won't accomplish much.
I actually think that the Democrats have done a pretty excellent job of communicating this theme. I mean, electing a black guy and a woman are pretty clear markers of what our coalition is all about. But it's naturally a difficult message to sustain and we have a coalition literally designed around people who don't show up for midterms. We just need to work on figuring out how to change that.