Vellum Pro by Florian Monnier

Vellum Pro repaints the Firefox interface in warm greys — yellow-brown rather than the blue-grey most themes reach for — with a tobacco accent. There are no background images, so nothing to download and nothing competing with your wallpaper.

Light and dark switch with your operating system, live, no restart. You can also pin either one from the preferences.

Hover, focus and selection are shown by fill, never by an outline. The active tab has no underline: it reads as a shape sitting above the tab strip. That puts all the weight on the value steps between surfaces, so there are three depth levels — soft, medium and strong — depending on your screen and how much separation you want. The preview in the preferences updates as you switch.

Private windows get a brick accent instead of tobacco, so you can tell them apart without reading the title bar. This needs "Run in Private Windows" enabled in about:addons.

Every text and icon colour was checked against every surface it can land on, across both schemes and all three depth levels. All pairs meet WCAG AA. Body text meets AAA everywhere except one: dark at strong depth, text on the focused address bar, which sits at 6.25:1.

Two things it deliberately does not do. It cannot change tab shapes, toolbar heights or the interface font — the Firefox theme API does not expose them, and the file that could is neither signable nor distributable. And forcing Light or Dark here does not change how web pages render; that setting lives in Firefox under Website appearance.

If you want a fixed palette with no moving parts, Vellum Pro Light and Vellum Pro Dark are the same colours as static themes.