docs: Add theme doc

docs/theme.md

@@ -0,0 +1,49 @@
+# How to Customize the Theme
+
+The components in this library come with a default set of colors and spacing.
+You can customize these properties (and add your own) by injecting a `Theme`
+object into the `BrandConfigProvider`.
+
+```TSX
+// App.tsx
+
+import { Theme, BrandConfigProvider } from '@lifeomic/react-native-sdk'
+
+const theme = new Theme({
+  colors: {
+    text: '#222222',
+    background: '#FFFFFF',
+  },
+  spacing: {
+    small: 10,
+    large: 28,
+  }
+})
+
+function App() {
+  return (
+    <BrandConfigProvider theme={theme}>
+      // ...
+    </BrandConfigProvider>
+  )
+}
+```
+
+You can also gain access to the default theme merged with your custom theme
+through the `useTheme` hook.
+
+```TSX
+// MyComponent.tsx
+
+import { useTheme } from '@lifeomic/react-native-sdk'
+
+function MyComponent() {
+  const { theme } = useTheme()
+
+  return (
+    <View style={{ backgroundColor: theme.colors.background }}>
+      // ...
+    </View>
+  )
+}
+```