Redesigns are tricky, because you can either get them right or alienate a good chunk of your userbase. Vivaldi is doing a major redesign of its browser, and it might change the way you use the browser a lot.
Vivaldi has just released a new testing 'snapshot' release packed with a bunch of changes. The most noticeable update is a refreshed user interface with a more spacious design, new icons, and updated background images. This modernization effort aims to keep the interface cohesive as new features are added. The redesign introduces floating tabs with a rounded shape, duo-tone icons for added depth, and a refined overall aesthetic.
You can further personalize your browsing experience with the UI density setting and a revamped theme editor. Some older themes have been retired, but you can still look for something you like in the Vivaldi Themes library. If you prefer the original look (redesigns are not always everyone's cup of tea and this could be true for you here too), it's still available as a "compact mode" in Settings.