The "Disable Sync" option background does not respect the Dark theme colors

[Softvision-MariusComan]

[Affected versions]:

• Firefox 57.0.4 and up

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest version of the "Firefox Notes" add-on (2.2.0dev custom build from 01.10.2018) installed.
• Have the "Dark" theme selected for the "Firefox Notes" add-on from about:addons->Extensions tab->Notes.
• You must be logged into "Firefox Notes".

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Click the "Notes" menu button.
3. Observe the "Disable Sync" option's background.

[Expected result]:

• The "Disable Sync" option has the same background as the "Give feedback" one.

[Actual result]:

• The "Disable Sync" option's background is much darker than the "Give Feedback" option's background.

[Notes]:

• The issue is not reproducible with the TxP version of "Firefox Notes".
• Attached a screen recording of the issue:
  

[Natim]

Fixes with #649

[Softvision-MariusComan]

This issue is no longer reproducible with the latest version of the "Firefox Notes" add-on (2.2.0rc5 build) installed, on the latest Firefox release and Nightly.