chore: add theme typings

Author: stipsan

src/sanity-ui.d.ts

@@ -0,0 +1,6 @@
+import { type Theme } from '@sanity/ui/theme'
+
+declare module 'styled-components' {
+  // eslint-disable-next-line @typescript-eslint/no-empty-object-type
+  interface DefaultTheme extends Theme {}
+}