Vivaldi was one of the first browsers to shake up the customization space when it was first released in 2015. Vivaldi came out of nowhere allowing you to customize just about every button along with full color themes, at a time when everyone was using the identical looking version of Chrome, Internet Explorer, and Safari.
This was the unique value proposition of Vivaldi back then, and seems to be the same to this day.
The problem is, when you allow for ultimate customization of every button and color theme, they seem to have locked themselves into an old and uninspiring design. One that looks like it did back in 2015. So at a time when modern solutions are coming out on a monthly bases, and customization and color theming is baked into just about every browser on the market, what once made it unique now leaves it in a questionable situation.