How to Increase the width of Vertical and Horizontal Scroll Bars in Ubuntu 18.04 LibreOffice

I wouldn't apply any changes to the stock theme. You can override the stock settings. With this method you cann still change the theme and keep your changes.

$ mkdir -p ~/.config/gtk-3.0/
$ touch ~/.config/gtk-3.0/gtk.css

Open the created file in the text editor of your choice and add these lines

scrollbar {
    min-width: 15px;
    min-height: 15px;
}

Both should have the same value. The one will be applied to horizontal the other to vertical scroll bars.

This method will change the width/height for all scrollbars and not just the once in LibreOffice

It turns out, according to this page, that LibreOffice doesn't use GTK 3.0, but GTK 2.0. That page was written in 2013, but it still appears to be the case for LibreOffice 5.x. I don't know whether things change with LibreOffice 6.x.

First, how to change LibreOffice apps, since that's the question: Edit (as root) /usr/share/themes/Mint-X [YOUR THEME HERE]/gtk-2.0/gtkrc:

GtkScrollbar::slider-width = 20 # or whatever pixel width you want.
# NB currently set at 11 in my setup

Second, how to change GTK 3.0 apps. One day LibreOffice may emerge from the primeval swamps and switch to it. As Eugen M.'s answer draws attention to, GTK 3.0 uses CSS (Cascading Style Sheets). According to my experiments, creating a file as he suggests has no effect on any apps, for the simple reason that he is not overriding the appropriate CSS class. This does work for me:

.scrollbar {
    -GtkRange-slider-width: 20;
}

This will in fact override the settings of your GTK 3.0 theme.

You have to override "widgets" CSS file at /usr/share/themes/Mint-X [YOUR THEME HERE]/gtk-3.0/gtk-widgets.css. So, you have to override the attribute of this name for .scrollbar.

NB: You are meant to be able to override your theme's GTK 2.0 settings by creating a file ~/gtkrc-2.0 (see here).