How to increase slider bar thickness (height, scale in Y)
3 Ways
Modify Scale Y of Render Transform
Increase Scale Y
Then decrease Size Y of Thumb
Then decrease Size Y of Slot
Result
Use ScaleBox
Put Slider into ScaleBox
Set Stretch to Scale to Fit Y
Change Horizontal Alignment and Vertical Alignment
Result
Or you can set Stretch to User Specified and Increase User Specified Scale
Hide background image (Thumb)
Then add an customized image down of Slider