Decision — style tokens as data

Status: accepted 2026-10-10.

The decision

• A typed style family in the design. The chrome family says which colours the chrome is. The style family says which forms it takes and how big they are. Every choice a renderer used to make by asking which scheme it was drawing is now one token:
  • forms, such as a tab strip or cards for panel groups, a checkbox or a switch for on/off controls, bevels or outlines, a ringed, plain or block slider knob, and pill or rounded push buttons;
  • lengths, such as the title-bar height, the menu row padding, the bar and dock sizes, the push-button height and the corner radii;
  • the renderer base: pair-based or chrome widgets, and a light, dark or mode-following base palette.
• Tokens are typed and closed. Each token is a named choice, a flag or a number with a declared range. Every style must author every token. A missing, unknown or ill-typed token is a compile error, as is a scheme with no style or one bound to a style that does not exist.
• Styles are named and bound per scheme. styles holds named token sets, and schemes gives every scheme exactly one of them. The shipped design has four styles: pro, studio and classic for the chrome schemes, and plain for the six hue schemes. plain writes down, as data, the geometry those schemes used to derive from their pair-based style. A style does not vary by mode or contrast, and modifier blocks cannot alter it.
• Renderers read the resolved style, never the scheme. The egui toolkit builds its chrome from the compiled style. No component branches on a scheme any more. A design that authors no style family takes the embedded design's style for its scheme.

Why

The chrome styles should be values a designer can edit and compare, not branches a programmer has to find. With the style family, a new style is a new token set in the design, with no renderer change. Moving every decision across changed no pixel: the toolkit's and the applications' snapshots render the same images before and after.

Consequences

• This lays the ground for a separate style axis, where any palette can be combined with any style. Today the binding from scheme to style is fixed in the design; a later decision can make the style its own selection.
• Scheme::is_chrome_scheme() now only groups schemes, for example in theme menus. The style's widgets token decides which styling path applies.
• A form the token set cannot express yet, such as a new tab shape, still needs code once: a new choice for the token and the code that draws it. After that, any style can select it.