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/Layan.colors
From under [Colors:Window] in Layan.colors, I used: BackgroundAlternate=54,56,62 for Vivaldi's Accent color, BackgroundNormal=49,49,58 for Vivaldi's Background color, DecorationFocus=86,87,245 for Vivaldi's Highlight color, and ForegroundNormal=234,234,234 for Vivaldi's Foreground color.
The background image is from: https://www.pling.com/p/1309218/