If it is a Mashup in Qlik Sense you are refering to you can make a small CSS-file with:
width: 15px !important; /*set your width and don't forget the "!important" to overide Sense own generated CSS client.css*/
there is also a ::-webkit-scrollbar-track and ::-webkit-scrollbar-thumb (for rounded corners etc.)
other then that I would refer to http://help.qlik.com/sense/2.0/en-US/developer/#Home-developer.htm
yes, that should be possible since every extension has a .css file connected to it.
but the name of the ID or class is possible another one. look for something with "scrollbar" in it etc.
Michael Tarallo wrote on how to find extesions:
This is quite out of the box, so i guess you would need some research before you start doing something like this.
I would suggest: