How to change the color of the scrollbar and slider in Kubuntu 20.4 systemwide [ Solved ]

Since you stumbled upon qtcurve in the question you linked, you just need to know, that you don't need to compile it yourself, in 20.04 the package exists in the official repositories and can be easily installed with

sudo apt install qtcurve

After installation, go to System Settings>Appearance>Application Style, choose the new application style "QtCurve" and edit it's settings.

After making your changes, close the editor window with the OK-button and then Apply your changes in the System Settings window.

Note that this will affect only native KDE applications which are based on Qt. It will not affect applications based on GTK.