Disabled sliders look odd with transparent button

[brs17]

Describe the problem: Creating an issue based off of the conversation had here: https://github.com/pop-os/gtk-theme/pull/409

When a slider is disabled, it looks odd for the button/selector of the slider to be partially transparent. I believe I find it odd because it feels like the button and slider are two different objects hacked together.

Steps to reproduce:

1. Open window with slider
2. Give a different window focus
3. Notice transparency

Distribution:

Pop!_OS 19.10

Pop version:

Any version including this patch: https://github.com/pop-os/gtk-theme/pull/409