I disagree. Perhaps it is simply a difference in definitions. To me a "theme" is a philosophy, or message that an author is intending to convey. Yes you can look at older works and point to things that are reflective of the moors or biases of the society of the time period, but that is not a theme. That is an influance.

The absense of asians in Shakespeare's writings are not indicitive of a secret bias he harbors against non whites, there simply weren't any in his society, so he didn't put them in his plays.