Started with the Vivaldi Human theme and kept the background image. From there, I used the colors to one of the images Substack frequently uses on their official posts (link in the Image source area). Rounded the corners to better correspond with Substack's look. If you'd rather invert the colors and have the Orange in the Toolbar instead of the Window, simply uncheck Accent on Window in the Theme Editor within Vivaldi's Settings. Use this theme as inspiration when you're posting content to Substack. Enjoy.