Based on vinceliuice's GPL-3.0 licensed GTK/KDE theme, Layan: https://github.com/vinceliuice/Layan-gtk-theme https://github.com/vinceliuice/Layan-kde
Under vivaldi://settings/appearance, enable "Use Native Window" for best effect with Layan GTK/KDE themes.
Colors from: https://github.com/vinceliuice/Layan-kde/blob/master/color-schemes/LayanLight.colors and https://github.com/vinceliuice/Layan-gtk-theme/blob/master/src/gtk/3.0/gtk.css
From under [Colors:Window] in LayanLight.colors, I used: BackgroundNormal=245,245,245 for Vivaldi's Background color, DecorationFocus=86,87,245 for Vivaldi's Highlight color, and ForegroundNormal=51,51,51 for Vivaldi's Foreground color.
From under headerbar in gtk.css, I used: background-color: #31313a for Vivaldi's Accent color.
The background image is from: https://www.pling.com/p/1309218/