x <- head(mtcars)
k <- knitr::kable(x, "html")

# First two rows bold and italic
row_spec(k, 1:2, bold = TRUE, italic = TRUE)

# Color one header background red with white text
row_spec(k, 
         row = 0, 
         background=c("","red",rep("",9)),
         color=c("","white",rep("",9))
         )

# Color columns of header alternating grey and green, via recycling
row_spec(k, 0, color=c('grey','green'))

# Color columns alternating grey and green in all rows, via recycling
row_spec(k, 0:nrow(x), color=c('grey','green'))

# Same thing using `column_spec`:
k |>
column_spec(seq(1, ncol(x)+1, by=2), color='grey') |>
column_spec(seq(2, ncol(x)+1, by=2), color='green')