Safari in its version 15 has not liked everyone. In fact, the system that Apple uses to manage the tabs in this new version has not been liked by almost anyone. One of the big problems you have is how to manage and know which tabs are active and which ones are not. Apple has released an update correcting some of the problems already reported by users, but without a trace of this change. But it can be achieved through a simple and inexpensive App that works as a browser extension.
The users of the new Safari are not especially happy with the way to manage the active tabs of those that are not in the browser. Apple has not made any changes in the tab shader neither in the beta version of Safari 15.1 nor in the latest version of the experimental browser Safari Technology Preview. But this does not mean that there is no solution.
To the user and developer Zhenyi Tan it occurred to you to use a simple Safari extension called ActiveTab which provides a solution, at least for now and until Apple can officially contribute its own. Although it may never do so, because you think there is no such problem. It’s all a matter of how many users complain about the shading. Going back to the solution used.
ActiveTab simply makes it easier to detect the active tab in Safari on Mac drawing a line under it. There are eight colors to choose from, and the line under the tab can be customized to be 1-7 pixels wide. The extension works best with the “Detach” tab layout selected and “Show color in tab bar” disabled in the Tab section of Safari Preferences. Zhenyi also cautions that ActiveTab won’t work well if you have too many tabs in one window.