For the first time in the modern era, non-Hispanic Whites are officially a minority in California, amounting to a little less than half the population of the state, down from nearly three-quarters a decade ago.

1. a little 是程度副词，不是代词，用在比较级前说明比较级的程度大小。跟 much 相对。我们常说：

a little more than，much more than

a little less than，much less than

a little older than，much older than

2. down from nearly three-quarters a decade ago：低于10年前的近四分之三。是对前文“人数略低于该州人口的一半”的进一步描述。修饰现在分词短语 amounting ...，看作补充性状语。