In desperation I have spent hours on the phone with Microsoft support for this issue and one other - but were left unsolved despite being on the phone for 2 hours.
I got to learn one thing, and that is the fading is hardcoded into Windows 8.1
So it seem it will not be solved unless someone write an actual hack for how the windows system behaves causing the scrollbar thumb to fade.
There's one add-on to get for Firefox which gives us a nice clear scroll bar thumb for the web.
But that does not solve the problem of not being able to see where on a page we are when working with documents - and get distracted by having to go chase after that scrollbar thumb to find the line where the data is to be changed in a spreadsheet or find a specific folder or document in explorer.
In my work environment I don't want any flashy 'themes' either, it distracts me from my work but just a discreet neutral coloured background on the desktop,
Neither that is an option in Windows nowadays, it appear they think computers are for children, computer gamers and that nobody actually use them to work any more. :(