Don't know what file to use to install a fix to ~/.config/gtk-3.0

How did you install the theme please?

I downloaded Clearlooks-Penix for gtk-3.20 from its web site (https://github.com/jpfleury/clearlooks-phenix), extracted it from its archive, changed the name of the extracted folder to Clearlooks-Phenix, and copied it to /usr/share/themes. I am running Xubuntu, which uses xfce. Then I went the Main Menu>Settings>Appearance>Style and set it to Clearlooks-Phenix. In the window that set up Appearance I selected "Icon" and set to Ubuntu-Mono-Light. I then went to Main Menu>Settings>Window Manager>Style and tried various different Styles there. I finally settled on Daloa. I could have used the Clearlooks-Phenix setting here also, but I preferred what I got using Daloa.

Also, I discovered that the correct file to use in ~.confix/gtk-3.0 is gtk.css, so my original question is answered. However, my problem as described in the first web site listed above, was still not solved so I experimented with different settings for the corrective code listed in that website. I discovered that the first setting is for slider trough space above the trough, the second for trough space to the right of the trough, the third for trough space at the bottom of the trough, the fourth for trough space to the left of the trough. Whenever, this trough space is clicked, the slider rapidly scrolls to the bottom of the page. So far the best setting I have found is "border-width: 0px 0px 0px 2px;". This still leaves a narrow space left of the slider that warp-speeds to the bottom of the page, but it removes the narrow space to the right of slider that warp-speeds to the bottom. I prefer having the problem space on the left rather than the right. I am going to experiment with other settings to try and get rid of the narrow warp-speed space on both sides of the slider.

If you experiment with this please keep me informed.......
Thank you very much for responding.

Why not just run:

sudo apt-get install clearlooks-phenix-theme

It’s in the repositories

That isn't the instructions for installing it. See the Clearlooks-Phenix website and the Readme.md file. But you are asking a good question. When I search for Clearlooks-Phenix in Synaptic, I see that I have clearlooks-phenix-theme version 7.0.1-1 installed, so I must have tried it at some point in the past. When I look at /usr/share/themes, I see that I have 3 clearlooks themes installed: Clearlooks.original (probably this was originally "Clearlooks" and I changed the name to Clearlooks.original), Clearlooks-Phenix (installed by copying it into /usr/share/themes), and Clearlooks XFWM4 (I am not sure where I got this). Any one of them could be the one listed in Synaptic.

I don't want to delete any of these various clearlooks themes, because I might damage my system. But if you install clearlooks-phenix from the repository, let me know what gets installed in /usr/share/themes. Then maybe I will clean mine up.

The files installed by the Ubuntu-provided package are listed on https://packages.ubuntu.com/bionic/all/clearlooks-phenix-theme/filelist