dark mode + slim mode do not affect the shell.

[nanopone]

Distribution (run cat /etc/os-release):
NAME="Pop!_OS"
VERSION="19.04"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Pop!_OS 19.04"
VERSION_ID="19.04"
HOME_URL="https://system76.com/pop"
SUPPORT_URL="http://support.system76.com"
BUG_REPORT_URL="https://github.com/pop-os/pop/issues"
PRIVACY_POLICY_URL="https://system76.com/privacy"
VERSION_CODENAME=disco
UBUNTU_CODENAME=disco

Related Application and/or Package Version (run apt policy $PACKAGE NAME):

Issue/Bug Description:
dark and slim options in appearance do not affect the shell. (volume and top bar are not themed)

Steps to reproduce (if you know):
turn on dark/slim on appearance in settings

Expected behavior:
volume is to be dark, top bar is to be slim.

Other Notes:

[isantop]

This is confirmed for now, because there is no way without enabling extensions to switch the shell theme on-demand, and the only extension that does allow it has a few issues that need to be resolved before we can consider including it by default.