Just FYI: In the original 16.04 release g-t's tabs looked like your sakura screenshot. It was a subsequent software update (dated perpaps 16.05 or 16.06) that changed g-t's look. (Not sure which package was responsible for the change, probably the theme.)
Hmm, like other comments already said it seems like something controlled by the theme. But maybe it can be done in sakura itself. For the time being i'll mark this bug as Confirmed. Thanks for the report.
Changed in sakura:
status:
New → Confirmed
summary:
- Make tabs more contrast + Make unactive tabs darker
Isn't it controlled by GTK theme? imgur.com/ a/1VTS6
In gnome 3 (Adwaita Black) it looks like here http://