Scale handles lose styling in backdrop

[isantop]

Describe the problem:

When their window is in :backdrop, the slider handles for GtkScale widgets lose their Pop styling.

Steps to reproduce:

1. Open Widget Factory
2. Open another window.
3. Without focusing, look at the scales in Widget Factory

Distribution:

PRETTY_NAME="Pop!_OS 19.10"

Pop version:

`Installed: 5.0.0~1573676952~19.10~8ddf0fc`