UPDATE 28 Feb 2015:
The prior method I listed no longer works as of Lubuntu 14.04, so use this method instead:
- Run this command in a terminal (or just navigate with admin privileges to this directory)
sudo leafpad /usr/share/themes/Lubuntu-default/gtk-2.0/gtkrc
- In the document that opens, find the line entry for:
GtkScrollbar ::slider-width = 9
- Replace the 9 with whatever number you want. I prefer a size of 20.
- Save the document
- Logout and Log-in. Done.
For history's sake, here are the older instructions, which no longer work:
Run this command in a terminal:
sudo leafpad /usr/share/themes/Lubuntu-default/gtk-2.0/scrollbar.rc
Edit this line in the resulting document that pops up:
GtkRange::slider-width = 20(replace "20" with your choice. 20 is a bit thick but that's how I like it. That's what she said)
Logout and Log-in again. Done.