The Scratch variable slider does not always harmonize with every project. Shape, size, colour are not freely customizable. A reason to complain? Certainly not! After all, Scratch provides you with everything you need to create your own personal slider.
So, have a look in some examples of this studio and learn from them and get some ideas!
Do you prefer a tutorial? Here you are: https://scratch.mit.edu/projects/300586242/
A tutorial on a related topic (scroll bar):
By the way, these and many other helpful tutorials can be found here: https://scratch.mit.edu/studios/3677618/
Did you create a pretty slider project? Let me know in the comments! There is still a lot of room in this studio!