The theme you want to focus on in response to this question is interracial marriage. Note the way that Rufus stridently states that black-white marriage is "against the law" in his world. What is ironic about this is that interracial marriage was only made legal in the Supreme Court's 1967 ruling of this case. In 1958, in Washington, Richard loving (white) and Mildred Loving (black) returned to Virginia. There, they were arrested in the middle of the night, being dragged out of their bed. The judge agreed to suspend their sentence of one year only if they agreed to leave the state of Virginia for 25 years and to never return together. Incensed by this rulings, the Lovings began to challenge this sentence, saying that this decision infringed the right to liberty and equal protection under the law. This case therefore was a very important ruling for the civil rights era. Of course, the way that Dana and Kevin's marriage has to be defended moments after their return to the past draws attention to the fact that had they been married only ten years earlier, their marriage would actually have been illegal. Butler therefore uses the theme of interracial marriage to draw attention to the way that some victories against racism and discrimination had only recently been won.

The novel Kindred, by Octavia Butler, is about Dana, a black woman,  her husband Kevin, a white man, and her heritage as a descendant from slaves.  Dana is the result of a family history which included her linage resulting from the rape of her black ancestor by a white slave owner.  The case of Loving vs Virgina is the decision by the U.S. Supreme court which allowed the marriage of people of different races, religions and national origin. 

The character, Dana, keeps being shifted back and forth in time to protect her ancestor Rufus Weylin.  Dana finds herself transported to 1815.  She is responsible for saving the life a Southern slave-owner and he becomes the future father of the first woman "listed in her family records—Hagar Weylin."

The US Supreme Court case Loving vs Virginia was in the 1960's and the novel is set in 1976.  Dana and Kevin are an example of the changes that had taken place in America since the decision to lift the ban of interracial marriages.  Butler juxtaposed this theme, of inter-racial marriage against the life of an African-American in the 1800s and the way the white owners treated their slaves.