Snooze Tabs icon and some of the text in the panels are wrongly colored in black

[SoftVision-CarmenFat]

[Affected versions]:

• Release 58.0.2 and up
• Snooze Tabs 1.1.0

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest Snooze Tabs add-on version (https://github.com/bwinton/SnoozeTabs/releases/tag/1.1.0) loaded as a temporary add-on.
• Default theme is enabled.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisite and navigate to a website.
2. Observe the "Snooze Tabs" icon from the toolbar.

[Expected result]:

• The icon is grey.

[Actual result]:

• The icon is black.

[Notes]:

• This issue is also reproducible with the icons in the context menu and in "about:addons" page and the "Manage Snoozed Tabs" text from the item's panel. "Pick a Date/Time" text from the same panel is also black instead of grey.
• Attached a screenshot of the issue: