The bar at the side of some open GUI programs has gone, it’s there in the browser but gone in programs such as Transmission.
How do I get it back in the ones that had it?
You probably changed something in gnome-tweak-tool
Which desktop ?
The one I’m using lol (only joking)
Gnome as I can’t stand the other one.
IIRC, there’s a setting in gnome-tweak-tool that changes the scroll bar from the new type (where scroll only appears when you hover the mouse over te righ of a window) to the old type (where the scroll bar is always present) … you didn’t change that did you ?
Don’t think I did, I couldn’t see that setting, unless I’m not understanding the name its called?
Just been looking through it and why is Shell Theme left blank?
Vertical scrollbars … try this:
Open Synaptic, and search for liboverlay-scrollbar
You should see some files listed with names like:
liboverlay-scrollbar-0.2-0
and
liboverlay-scrollbar3-0.2-0
(make a note of the names in case you want to replace them)
If any of them are installed (green box) … mark them for removal, click “Apply” (to remove them) … REBOOT, to apply the changes.
That should get you back to the original scrollbar style.
[EDIT]
Are you using Gnome shell, or Gnome Classic ? … the themes don’t work in “Classic” … and IIRC, you have to install some themes first anyway.
sorted, it’s back to normal 
Can you mark this topic as (SOLVED) … byt editing the FIRST posting in the topic, and changing it’s “Subject” line from:-
Slider bar gone
to
Slider bar gone (SOLVED)
This makes it easier for future members to search for a solution … TIA