Mint-X theme puts a red border around some windows

[Four]

This is on Linux Mint 20.1, Mate. With the Mint-X theme, I get a bright red border around windows of certain applications. It doesn't happen with Mint-Y.

It seems specific to Gnome apps, which don't have the normal kind of title bar. It happens for these:
Disks
dconf Editor
Gedit

This post on Reddit shows the exact same thing, but no solution:
[url]https://www.reddit.com/r/linuxmint/comments/9k3nbb/mint_19_mate_red_outline_around_document_viewer/[/url]

[Four]

It's in this file, in the section for csd:

/usr/share/themes/Mint-X/gtk-3.0/gtk-widgets.css

Code: Select all

 .solid-csd decoration {
    margin: 0;
    padding: 4px;
    background-color: @border;
    border: 1px solid @border;
    border-radius: 0;
    box-shadow: inset 0 0 0 3px red;
 }

Setting each of those px widths to 0 gets rid of it.

The csd is for client-side decoration, https://en.wikipedia.org/wiki/Client-side_decoration, so it makes sense that it is on the Gnome apps.