Arc Darker is my favourite Gnome theme, and as a bonus using this Firefox theme removes the buggy white dots at the top corners when using 'no titlebar' option in Firefox 61 with Gnome 3.28 and Arc Darker GTK3 theme.

Looks better than the dark and white theme, which comes pre-installed with firefox. Refined looks, Thank you. Would love to have the same the with 100% black tab bar.