The text on Firefox Screenshots UI buttons is invisible when using dark theme

[SoftVision-CosminMuntean]

This issue is cloned from Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1493511

I'm using a dark UI theme for everything on Linux, and I've noticed that under these circumstances, the text is invisible on the three buttons which appear when I choose to make a screenshot using Firefox Screenshots.

Attaching a screenshot manifesting the issue. I actually thought this was my locale lacking the strings, until I looked at the buttons very closely and actually saw that there is text on them, which is basically invisible.

[SoftVision-CosminMuntean]

Another similar issue was logged Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1513898

The reporter reproduced the issue on Kubuntu 18.04.1 desktop with a dark theme (Breeze Dark).