docs: add styles to MCP Apps specification

[antonpk1]

Summary

Adds documentation for the styles field introduced in #127 to the MCP Apps specification (SEP-1865).

• Add styles field to HostContext interface
• Add dedicated Theming section documenting all 36 standardized CSS variables
• Document Host/App behavior including light-dark() usage
• Add Design Decision docs: update UI resource format and metadata fields #4 in Rationale section explaining the approach

Changes

HostContext Interface

• New styles?: Record<string, string> field with reference to Theming section

Theming Section

• Table of 36 CSS variables organized by category (colors, typography, borders)
• Host Behavior: MAY provide any subset, MAY use light-dark()
• App Behavior: SHOULD use fallbacks, MUST set color-scheme when using light-dark() values

Rationale

• Design Decision docs: update UI resource format and metadata fields #4 explains why CSS variables, limited set, no spacing, no component library

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npm i https://pkg.pr.new/modelcontextprotocol/ext-apps/@modelcontextprotocol/ext-apps@130

commit: 854fec2

[liady]

@antonpk1 looks like a good subset of variables.

1. Do we want to state that apps MUST apply these variables? Currently it's not mentioned explicitly in the SEP.
2. Do we think in the future we'll add more style customization mechanisms? Perhaps the key should be more specific, like styleVariables - or a key in anobject (styles.variables).
3. Do we also want more semantic variables and not just these style atoms? For example if we want buttons to be consistent across apps (one app can use border-radius-small for buttons and other can use border-radius-medium). I think we need to decide the level of design consistency we're aiming for across apps - which might require more semantic definitions (or a concerete example).

[martinalong]

Merging this into my PR, but @liady I'll address your points in my PR!