[tweak] dark mode colors (#2469)

This PR fixes a few dark mode colors and removes some unused styles.

### Change Type

- [x] `patch` — Bug fix

scripts/purge-css.ts

@@ -0,0 +1,14 @@
+import { PurgeCSS } from 'purgecss'
+import { nicelog } from './lib/nicelog'
+
+async function main() {
+	const purgeCSSResults = await new PurgeCSS().purge({
+		content: ['packages/tldraw/**/*.tsx', 'packages/editor/**/*.tsx'],
+		css: ['packages/tldraw/src/lib/ui.css'],
+		rejected: true,
+	})
+
+	nicelog(purgeCSSResults)
+}
+
+main()