NoMore404s icon from the browser toolbar, is hardly visible on darker themes

[SoftVision-CosminMuntean]

[Notes]:

• The NoMore404s icon is also hardly visible on Firefox High Contrast themes and other 3rd party darker themes.

[Affected versions]:

• Aurora 53.0a0
• NoMore404s 1.7.1

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Latest NoMore404s add-on (v1.7.1) was previously installed on a new profile.

[Steps to reproduce]:

1. Open the Developer Edition bowser with the profile from prerequisite.
2. Observe the NoMore404s icon from the browser toolbar.

[Expected result]:

• The NoMore404s icon has the same color as the others icons from the browser toolbar and is perfectly visible.

[Actual result]:

• The NoMore404s icon is hardly visible.

[Additional Notes]:

• Attached a screen shot with the issue:

[marsjaninzmarsa]

Related to bwinton/SnoozeTabs#67.

[audreytoskin]

Setting an alternate icon for a dark browser theme would be nice, but you could also stick with a single icon image if it were filled with a solid color. Right now it looks like the image of a library building is transparent, "cut" out of a solid dark background. Just color the library white, and it will still be visible.