Make the DocPress design swappable without forking

Following the palette PR (#171): colors are now tokenized. The bigger goal is making the whole look swappable, so a redesign is a preset/config swap rather than a fork that one person owns. vike.dev stays byte-identical by default.

**The constraint**
vike-themes and DocPress don't compose at the layout level: each owns its own shell/renderer (DocPress isn't vike-react, so the vike-themes Wrapper hook doesn't apply). Today they only meet at the CSS-variable seam.

**Approaches to weigh (nothing decided yet)**
- Extend the CSS-var seam to layout: tokenize type scale, spacing, radii, a few layout dials (nav width, max-widths). Incremental, same shape as #171.
- A different mechanism: layout slots, deeper shell integration, or something else entirely. Open to better ideas.

Goal here is to agree the approach before building. No rush.

Start by reviewing palette PR #171 and the current CSS-variable seam. Compare how vike-themes and DocPress own their shell and renderer, then evaluate the proposed layout-token, slot, or deeper-integration approaches. Done means agreeing on an approach that makes the design swappable while keeping vike.dev byte-identical by default.

Written by the indexing model from the issue text.