Styling scrollbar thumb

Hi,

I am customizing my scrollbar to look like the one I designed in my software (see images).
scrollbar2

The problem is that I can’t get my thumb to be this little yellow square because it won’t change size. I’ve used various forums, sites, customizers, and ChatGPT, but nothing works. I feel like it’s not possible and I might have to cheat by creating a fake scrollbar.

Do you have any solutions to my problem?

Thank you!