Fix for custom theme (has no css rules)

Author: tohosaku

src/core.js

@@ -43,7 +43,9 @@ export class JSONEditor {
       /* Attempt to locate a shadowRoot parent (i.e. in Web Components) */
       const shadowRoot = getShadowParent(this.element)
       addRules('default', rules, shadowRoot)
-      addRules(themeName, themeClass.rules, shadowRoot)
+      if (typeof themeClass.rules !== 'undefined') {
+        addRules(themeName, themeClass.rules, shadowRoot)
+      }
     }
 
     /* Init icon class */

tests/unit/core.spec.js

@@ -78,6 +78,13 @@ describe('JSONEditor', function () {
     expect(editor.theme).toBeTruthy()
   })
 
+  it('can add custom theme (no custom css rules)', () => {
+    class CustomTheme extends JSONEditor.AbstractTheme {}
+    JSONEditor.defaults.themes.myCustom2 = CustomTheme
+    editor = new JSONEditor(element, { schema: schema, theme: 'myCustom2' })
+    expect(editor.theme).toBeTruthy()
+  })
+
   it('can add custom editor', () => {
     class CustomEditor extends JSONEditor.AbstractEditor {}
     JSONEditor.defaults.editors.custom = CustomEditor