From "The Tragedy of Safari 15 for Mac’s ‘Tabs’" posted Friday on Daring Fireball:
Our long national iOS 15 Safari nightmare ended last month, praise be, but the lesser of the two bad Safari designs unveiled at WWDC persists and actually shipped: the new tabs in Safari 15 for Mac. Safari 15 on iPad suffers similarly, but it’s the Mac version I’ll concentrate on here.
The most controversial Mac Safari changes shown at WWDC — compressing tabs and the URL location field into a single row at the top of each window, and coloring the entire window with the accent color of the currently frontmost web page — are settings that (thankfully) can be turned off in Safari’s Preferences window (under “Tabs”, natch). The “Compact” layout that puts tabs and the location field in the same row — by using the tabs themselves as the text editing fields for URLs — is, thankfully, off by default. But the “Show color in tab bar” option is on by default... [see above]
My take: First thing I did was turn it off.