Snap 'ed applicaitons have garbage on top of the window Close [x] button

Steps to reproduce:
1. Have Ubuntu 19.10 installed
2. Install GNOME application as a Snap - for example with

    sudo snap install gedit gnome-calculator

3. Open GNOME Tweaks -> Appearance tab, then switch Icon set to the one of the following:

* Adwaita
* DMZ-Black
* DMZ-White
* hicolor
* HighContrast
* Humanity
* Humanity-Dark
* LoginIcons

For example - Adwaita theme with Adwaita icon set.

4. Open GNOME Calculator as a Snap application:

   /snap/bin/gnome-calculator

Expected result:

* GNOME Calculator looks as any other GTK application

Actual result:

* GNOME Calculator has garbage on top of the window Close [x] button

---

Notes:
1. Problem was first seen on AskUbuntu - https://askubuntu.com/q/1206868
2. one can use the script below to check all possible Theme<->Icon combinations:

```
#!/bin/bash

DISPLAY=:0

for t in $(ls -1 /usr/share/themes/)
do
 for i in $(ls -1 /usr/share/icons/)
 do

  echo -e "\nTheme: $t, Icons: $i\n"

  gsettings set org.gnome.desktop.interface gtk-theme "$t"
  gsettings set org.gnome.desktop.interface icon-theme "$i"

  /snap/bin/gnome-calculator

  #read -r -s -p "Press <Enter> for next theme"
 done
done

```

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: snapd 2.41+19.10.1
ProcVersionSignature: Ubuntu 5.3.0-29.31-generic 5.3.13
Uname: Linux 5.3.0-29-generic x86_64
ApportVersion: 2.20.11-0ubuntu8.2
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat Feb 1 17:55:09 2020
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: snapd
UpgradeStatus: No upgrade log present (probably fresh install)