How to input the opposite strand of DNA?
Yes, you're going to want to take the reverse complement of your string in order to analyze the other strand, that is the standard.
Also, it is worth pointing out that you may want to use a generator expression instead of a list expression for your chunk_string function:
def chunk_string(s, n):
    return (s[i:i+n] for i in range(len(s)-n+1))

This will result in a generator that yields kmers as the Counter constructor iterates through them, which means that you don't need to first construct and store the entire list of kmers first and then pass to Counter but instead can feed them directly to it. Not essential for small strings, of course, but a good practice and useful for larger strings. See this SO post for a better explanation.
You can also use Biopython's reverse_complement function in order to easily obtain the reverse complement.