My two cents.
First, what you mention in terms of similarities in style of fighting in a population (darker Thais and African Americans / Cubans respectively) are better explained by classifying the styles as regional rather than having to do with genetics. If a region is predominantly inhabited by people of the same ethnic background however, it might SEEM to be that their style is linked to race. However, looking at a 'similar' population in different regions suggest that attributing styles to regions instead of race or ethnicity seems a much clearer explanation of why certain groups tend to exhibit certain styles.
For example, African Americans and Afro-Cubans are mentioned as slick, but Ghanaian boxers (Ghana arguably has the most well-established boxing scene in West Africa) tend to not be slick.
In Myanmar ,Lethwei fighters (even those from the border region with northern Thailand who are likely to be similar to darker skinned Thais genetically) are typically not deemed slick.