Token updates: surfaces, radii, borders, semantic spacing

[ryankeairns]

Part of #9893

Goal

Theme/token work only — no component behavior changes yet (except where a token change is the whole deliverable).

Surfaces

Darken the main background for clearer contrast under the workspace, and add one optional secondary surface for inner steps (dashboard, secondary nav).

• Change main/canvas background — stronger contrast under the workspace (POC light: Borealis #F6F9FC → ~#EAEDF5; dark TBD)
• No change to plain, raised app/workspace (#FFFFFF / plain)
• Add secondary surface token for inner steps (e.g. dashboard, secondary nav) — POC ~#F8F9FB, or reuse subdued

Borders & elevation

Prefer flat surfaces with gaps and quieter edges over heavy dividers and drop shadows; soft elevation only where needed (e.g. app workspace).

• Quieter edges (open) — keep real 1px borders (optionally quieter color on borderBaseSubdued / borderBasePlain), or move edged surfaces to a faux border (box-shadow: 0 0 0 1px …). Pick one system; don’t ship both as parallel tracks. See epic [Draft][Epic] Modernize Kibana visual language — EUI tokens & component defaults #9893 open decision.
• Use borders and dividers more sparingly — fewer internal dividers is usage/guidance, not a new token; keep dividers in redesigned input controls (appends/prepends)

Radii

Move off Borealis’s near-uniform ~4px toward clearer tiers — sharper on controls, softer on containers.

• Control / form — 8
• Panel — 12
• Container — 16
• Full/pill — TBD for filled/primary CTAs; probably not a general radius token

Semantic spacing

Add intentional spacing.* aliases that map to existing size.* values — so usage says what the space is for, not a generic value. Keep raw size.* as the scale.

• Stay on-scale — prefer 8 / 12 / 16 / 24; avoid off-scale values like ~20 unless intentional

Token	Maps to (propose)	Notes
spacing.panelInset	size.base	EuiPanel / container padding
spacing.pageInset	size.l	Page / header padding
spacing.controlsGap	size.s	Space between related controls

Dashboard / layout grid gaps stay product-side (not an EUI gutter.* token).

Typography

Moved to #9901 (component defaults + title letter-spacing).

Out of scope

Component default flips (panels, buttons, forms, tabs, tables) — see sibling issues.

Open

See epic #9893 open decisions (theme vehicle, quieter edges / faux border, third surface name).