Thanks for reaching out in our design forum! This is the place to go when it comes to coding advice where you can hopefully connect with someone with insight regarding your particular request provide you with the necessary code for adjusting the divider line widths.
With that said, we are not formally trained in code to be able to assist you directly but I just wanted to leave a valuable alternative solution if you are unable to get an answer in the near future. Shopify has an experts platform that allows you to find and hire a developer for small and large coding gigs. With the help of an expert, they could definitely help you do this at a modest cost. If you would like to learn more, feel free to check out our Shopify experts directory.
If there is anything else I can help you with, please let me know.