chore: add CSS ADR

docs/architecture/003-css-framework.md

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+# CSS Framework Selection
+
+## Context
+
+The current CSS framework, [Stitches](https://stitches.dev/), is no longer maintained, and we are facing limitations due to the inability to use a runtime CSS-in-JS framework with React 18's server components and the Next.js app router.
+
+## Decision Drivers
+
+* Similarity of new framework API to Stitches, both for consumer onboarding and automated conversion
+
+* Ability to work with our current tooling. Storybook for development, tsup for compilation, and Next.js for production applications
+
+* Ability to work with other styling options in current use including Tailwind. 
+
+## Decision
+
+We propose adopting Panda CSS, a compile-time CSS-in-JS framework, as the new CSS framework for WPDS.
+
+We will continue to maintain a Tailwind theme that can be used in conjunction with our components for teams that prefer a utility class based approach to styling.
+
+## Considered Options
+* [Vanilla Extract](https://vanilla-extract.style/): Considered for its approach to type-safe styles and compile-time generation of stylesheets.
+  * Uses external *.css.ts breaking our current colocated styles
+  * Does not have a compatible plug-in for Next 13.x or 14.x
+  * Documented integration with Storybook is broken
+  * Approach to variants is significantly different from Stitches adding to level of effort
+
+* [StyleX](https://stylexjs.com/): Examined for its runtime performance, support for dynamic theming, and expected long-term support from Meta.
+  * No plugin is available for esbuild or tsup forcing a significant change to our compilation process
+  * Override styles must use `stylex.create` , both className and style are actively discouraged or prevented. Generated classes are high strength and difficult to override
+  * Approach to themes and variants is significantly different from Stitches adding to the level of effort
+
+## Resources
+[Panda CSS Spike](https://arcpublishing.atlassian.net/browse/SRED-318)
+[Vanilla Extract CSS Spike](https://arcpublishing.atlassian.net/browse/SRED-317)
+[StyleX CSS Spike](https://arcpublishing.atlassian.net/browse/SRED-643)
+
+## Scope
+
+Creating a new WPDS Theme formatted for Panda derived from existing tokens. Migrating components using Stitches to Panda CSS. Both these constitute breaking changes that would mean a 3.0 release of WPDS
+
+Once complete product teams can then begin to opt in to 3.0
+
+Tachyons is deprecated and will no longer receive updates, teams wanting to use utility classes should use Tailwind
+
+## Status
+
+Proposed