Default Dark Theme Tab Color Contrast
If you select the dark theme (no red) during installation, the tabs all look the same, the active tab should be a different color so it's easy to see.
On my system (Windows 10) the active tab is a medium gray, while all of the other tabs are nearly black. I suppose there could be more contrast, and even more visibility, but it's quite usable here.
In Windows 7 there wasn't much difference, had a hard time looking for the active tab.
At the moment, Vivaldi is coloring the tab bar based on the colors of the current page.
In future versions they could add the option to select custom colors (right-click -> customize).