Read Next address bar widget can have wrong colour on certain themes

[Aleksandra0056]

I'm using the Activist – Soft theme by Mozilla, but every time I look at the address bar icon of Read Next, I notice clearly, that the icon uses grey colour instead of black (black is the standard colour of button icons on Activist – Soft).

Screenshot:

My theme website:
https://addons.mozilla.org/en-GB/firefox/addon/activist-soft/