How do I make the scrollbars wider?

This gets about half of them:

Add the following lines to ~/.gtkrc-2.0

style "scroll"
{
    GtkScrollbar::slider-width        = 40
}

class "*" style "scroll"

It works for firefox, kicad, and several others.

And this seems to get the other half:

For the rest (the gtk3 stuff, I guess), I copied the theme I was using from /usr/share/themes into ~/.themes and gave it a new name, "MyTheme". Then in ~/.themes/MyTheme/gtk-3.0, I found the css file containing "-GtkScrollbar-slider-width:" and changed the value there. I used the gnome tweak tool to select my new theme. Now my sliders are all 40 pixels wide!

I'm using Ubuntu 15.10. Here's how I increased my scrollbar width:

I'm using the Ambiance Theme, so this is what I did:

Edit CSS file using this command:

sudo gedit /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css

Look for this section:

/*************
* scrollbar *
*************/
.scrollbar {
-GtkScrollbar-has-backward-stepper: 0;
-GtkScrollbar-has-forward-stepper: 0;
-GtkScrollbar-trough-border: 0;
-GtkScrollbar-min-slider-length: 31;
-GtkRange-slider-width: 30; <------------------Change This!!!

Then save the file, and logout/login again.

I was so frustrated with the annoying super-narrow scrollbars, that I almost switched back to Windows over this issue. The Ubuntu designers really screwed this up. Usable scrollbars are kind of important. Making them super narrow to take up less space was not a good idea. They need to be large so they're easy to click on! Ubuntu, like Microsoft, is trying so hard to make things look good on Mobile that they just end up ruining things for non-mobile users. (Forgive me for the critique)

First of all, you should look for which theme you are using. In my case, for Ubuntu 17.10 the theme was Ambiance, and the concrete file to edit was this one:

/usr/share/themes/Ambiance/gtk-3.20/gtk-widgets.css

Look for this part of the CSS file:

.scrollbar.vertical slider,
scrollbar.vertical slider {
    min-height: 15px;
    min-width: 5px;
}

And then change the min-width: 5px; for whatever value you think is great. I changed it to 12px and it is incredible :)