Update CSS-related README

src/styles/README.md

 # Styling components in the client application
 
-This application is transitioning how CSS styling is applied.
+This application is primarily styled by inline Tailwind utility-first classes.
+Where necessary and sensible, external CSS may be used.
 
-- OLD: BEM-structured SASS, using mixins, partials and variables
-- NEW: Utility-first styling in components using Tailwind
-
-At present, you'll see a mix of legacy SASS and Tailwind. The task at hand is to finish converting legacy styles and assuring that new styling follows the structure outlined here.
-
-- Reference [Tailwind documentation](https://tailwindcss.com/docs/installation).
-- Components should use Tailwind utility classes in source JSX — exclusively, where possible.
-- External CSS, where needed, should be put into the Tailwind `components` layer and use `@apply` directives to reference applicable utility classes. See the `Buckets` annotator component and its styles for examples.
-- Tailwind is configured by this project's `tailwind.config.js`, which extends the [Tailwind preset provided by the `@hypothesis/frontend-shared` package](https://github.com/hypothesis/frontend-shared/blob/main/src/tailwind.preset.js) This configuration determines what utility classes are available. Add new needed Tailwind configuration properties as needed[^1] and consider extracting common properties to the `frontend-shared` package.
-- Check for unused SASS variables and mixins — and remove them.
-
-What we're aiming for at the end of this conversion process:
-
-- No SASS variables or mixins
-- External CSS minimal and within appropriate Tailwind layers
-
-[^1]: You will need to restart the local dev server after making configuration changes
+See https://github.com/hypothesis/frontend-toolkit/blob/master/docs/css-style-guide.md