I started with the Breeze-Dark plasma theme. This is derived from the Breeze plasma theme but the theme color isn't adaptive (dark/light).
But I made some changes:
- this plasma theme has straight corners
- made the panel a little more transparent and changed the color a little
- color of the accents within the theme will now follow the system accent color
- removed almost all content so it will be replaced by the "default" plasma theme's elements
- added another analog clock widget

I made a version with rounded corners too: vesuvius https://www.pling.com/p/1867953/
Icons used on images on top: https://www.pling.com/p/1873402/
Two images on top: 1) desktop Plasma, 2) detailed view of the panel