Hello JonC,
Thanks for submitting a question about coding with CSS. I’d recommend using the developer tools in your browser to make sure you are targeting the precise CSS selector. Using these tools will allow you to test your code live so you can be sure it takes effect as desired.
We’re unfortunately not equipped to provide individual coding solutions upon request via this public forum. It is highly likely a custom solution designed specifically for you by a professional WordPress theme designer would be a more optimal and longer term solution in either case.
Best,
Christopher M.