There's no setting. In the latest version the tabs use standard Windows dark theme, which has (slightly) different color or active and non-active tabs (the same way it has slightly different colors in the light mode).
The update is a welcome improvement. A clear active tab color makes a big difference, especially when several tabs are open in dark mode. It would be helpful to have a simple setting to choose the highlight color as well.
Upgraded to 6.7, thanks for the suggestion.
And the color setting is ... where?
Windows 11, dark mode. Many tabs open. It is indistinguishable which one is the active because everything is black, the tab, the menu, the content and adjacent tabs.
Active tab color should be set in dark mode with some red frame or something.