make useTheme function more streight forward

Author: kof

packages/react-jss/.size-snapshot.json

@@ -1,23 +1,23 @@
 {
   "react-jss.js": {
-    "bundled": 169886,
-    "minified": 59592,
+    "bundled": 169726,
+    "minified": 59567,
     "gzipped": 19571
   },
   "react-jss.min.js": {
-    "bundled": 112526,
-    "minified": 42792,
-    "gzipped": 14525
+    "bundled": 112366,
+    "minified": 42767,
+    "gzipped": 14528
   },
   "react-jss.cjs.js": {
-    "bundled": 27813,
-    "minified": 12346,
-    "gzipped": 3868
+    "bundled": 27659,
+    "minified": 12314,
+    "gzipped": 3863
   },
   "react-jss.esm.js": {
-    "bundled": 25396,
-    "minified": 10449,
-    "gzipped": 3643,
+    "bundled": 25242,
+    "minified": 10417,
+    "gzipped": 3635,
     "treeshaked": {
       "rollup": {
         "code": 1815,

packages/react-jss/src/createUseStyles.js

@@ -18,22 +18,24 @@ import getSheetClasses from './utils/getSheetClasses'
 
 const useEffectOrLayoutEffect = isInBrowser ? React.useLayoutEffect : React.useEffect
 
-const noTheme = {}
+const noTheme: {} = {}
 
-type CreateUseStyles = <Theme: {}>(Styles<Theme>, HookOptions<Theme> | void) => any => Classes
+type CreateUseStyles = <Theme: typeof noTheme>(
+  Styles<Theme>,
+  HookOptions<Theme> | void
+) => any => Classes
 
-const createUseStyles: CreateUseStyles = <Theme: {}>(styles, options = {}) => {
+const createUseStyles: CreateUseStyles = <Theme: typeof noTheme>(styles, options = {}) => {
   const {index = getSheetIndex(), theming, name, ...sheetOptions} = options
   const ThemeContext = (theming && theming.context) || DefaultThemeContext
 
-  /* eslint-disable no-unused-vars */
-  const useTheme =
-    typeof styles === 'function'
-      ? // $FlowFixMe[incompatible-return]
-        (propsTheme?: Theme): Theme => propsTheme || React.useContext(ThemeContext) || noTheme
-      : // $FlowFixMe[incompatible-return]
-        (_?: Theme): Theme => noTheme
-  /* eslint-enable no-unused-vars */
+  const useTheme = (theme?: Theme): Theme | typeof noTheme => {
+    if (typeof styles === 'function') {
+      return theme || React.useContext(ThemeContext) || noTheme
+    }
+
+    return noTheme
+  }
 
   return function useStyles(data: any) {
     const isFirstMount = React.useRef(true)