Icon colors on Firefox with dark theme

[peter-lyons-kehl]

When in Firefox with dark theme (and dark toolbar), the Docs After Dark's toolbar icon extension is almost invisible (especially so in "compact" mode and/or on high resolution screens). Here's a screenshot (right of "Aa", which is another light-on-dark extension):

Please crate alternative icon/image for dark Firefox (and probably other browsers, too). See https://addons.mozilla.org/en-US/firefox/addon/tranquility-1 as an example.

This is useful especially because this extension is for evening/dark, or for people who like the dark background.