fix: make properties in theme prop optional

this doesn't work right now, but should work from flow 0.57 onwards

Author: satya164

docs/pages/2.theming.md

@@ -48,7 +48,7 @@ export default function Main() {
 
 You can change the theme prop dynamically and all the components will automatically update to reflect the new theme.
 
-A theme contains the following properties:
+A theme usually contains the following properties:
 
 - `dark` (`boolean`): whether this is a dark theme or light theme.
 - `roundness` (`number`): roundness of common elements, such as buttons.
@@ -60,7 +60,7 @@ A theme contains the following properties:
   - `text` - text color for content.
   - `disabled` - color for disabled elements.
   - `placeholder` - color for placeholder text, such as input placeholder.
-- `fonts` (`object`): various fonts used throught different elements
+- `fonts` (`object`): various fonts used throught different elements.
   - `regular`
   - `medium`
   - `light`

src/core/withTheme.js

@@ -10,13 +10,15 @@ const isClassComponent = (Component: Function) => !!Component.prototype.render;
 
 export default function withTheme<Props: {}>(
   Comp: React.ComponentType<Props>
-): React.ComponentType<$Diff<Props, { theme: Theme }>> {
+): React.ComponentType<
+  $Diff<Props, { theme: Theme }> & { theme?: $Shape<Theme> }
+> {
   class ThemedComponent extends React.Component<*> {
     /* $FlowFixMe */
     static displayName = `withTheme(${Comp.displayName || Comp.name})`;
 
-    _previous: ?{ a: Theme, b: ?Theme, result: Theme };
-    _merge = (a: Theme, b: ?Theme) => {
+    _previous: ?{ a: Theme, b: ?$Shape<Theme>, result: Theme };
+    _merge = (a: Theme, b: ?$Shape<Theme>) => {
       const previous = this._previous;
 
       if (previous && previous.a === a && previous.b === b) {