Go to OSX Daily to read How to Turn Off Tab Bar Colors in Safari for Mac
The latest versions of Safari on Mac apply a color tint effect to the tab toolbar. This shifts the browser window towards the color of the webpage in view, giving it a sort of transparent look. The color effect applies to the area of the search / URL bar, back/forward buttons, tabs, bookmark buttons, and the entire general top of a Safari window on a Mac.
Sometimes the Safari tab bar coloring can be extremely garish or just distracting, but if you’re not a fan of the appearance, you can disable the Safari tab color effect easily in Safari for Mac.