Unpinned sidebar is wrongly transparent on Windows High Contrast themes

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 52 and above

[Affected Platforms]:

• All Windows

[Prerequisites]:

• The "xpinstall.signatures.required" is set to "false" and latest Tab Center add-on is installed on a new clean profile.
• High Contrast #1 Windows theme is enabled (Start Menu -> Settings -> Themes -> Theme settings)

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to www.reddit.com.
2. Click on the "Shrink sidebar when not in use" button.
3. Observe the Sidebar.

[Expected result]:

• The Sidebar is opaque.

[Actual result]:

• The Sidebar is transparent and the content of the website is visible when it's expanded.

[Notes]:

• This is also reproducible if High Contrast #2 and High Contrast Black themes are enabled.
• This is not reproducible if High Contrast White theme is enabled.
• Please see the Screen Cast attached:

[ericawright]

I believe this is the case on all themes on all OSs. I can reproduce on mac in a standard dark theme. I'm not entirely sure this is not intentional?