Snap theming broken in Wayland

[Ads20000]

Reporting as per https://discourse.ubuntu.com/t/the-state-of-wayland-vs-x-esp-in-regard-to-to-the-next-lts/12576/15?u=ads20000

LibreOffice and System Monitor are Yaru in Xorg but are Ambiance in Wayland. Firefox is Adwaita in Wayland... I'm on Ubuntu 19.04, don't know if this also applies to 19.10 and 20.04.

```
$ snap version
snap 2.42.5
snapd 2.42.5
series 16
ubuntu 19.04
kernel 5.4.6-050406-generic

$ snap info core
tracking: stable
refresh-date: 16 days ago, at 10:14 GMT
channels:
  stable: 16-2.42.5 2019-12-12 (8268) 93MB -
installed: 16-2.42.5 (8268) 93MB core

$ snap info libreoffice
tracking: candidate
refresh-date: 10 days ago, at 14:02 GMT
channels:
  stable: 6.3.4.2 2019-12-18 (163) 439MB -
  candidate: 6.3.4.2 2019-12-18 (163) 439MB -
installed: 6.3.4.2 (163) 439MB -

$ snap info gnome-system-monitor
tracking: stable/ubuntu-19.04
refresh-date: 8 days ago, at 23:40 GMT
channels:
  stable: 3.32.1-3-g0ea89b4922 2019-12-20 (123) 3MB -
installed: 3.32.1-3-g0ea89b4922 (123) 3MB -

$ snap info firefox
tracking: stable
refresh-date: 23 days ago, at 09:43 GMT
channels:
  stable: 71.0-5 2019-12-03 (287) 226MB -
installed: 71.0-5 (287) 226MB -
```

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: snapd 2.42.1+19.04
Uname: Linux 5.4.6-050406-generic x86_64
ApportVersion: 2.20.10-0ubuntu27.3
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat Dec 28 22:47:28 2019
InstallationDate: Installed on 2017-08-03 (877 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: snapd
UpgradeStatus: Upgraded to disco on 2019-05-22 (220 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Michael Haworth (bailtree)]

I can confirm this is an issue on a 20.04 daily amd64 install. On X11, the theme is set correctly but the cursor is not the same as the cursor I'm used to (I noticed this in the discord snap app too)