fix(theming): more visual bugs (#34987)

(cherry picked from commit 569a7b33a5)

superset-frontend/packages/superset-ui-core/src/components/AsyncAceEditor/index.tsx

@@ -393,10 +393,7 @@ export const FullSQLEditor = AsyncAceEditor(
   },
 );
 
-export const MarkdownEditor = AsyncAceEditor([
-  'mode/markdown',
-  'theme/textmate',
-]);
+export const MarkdownEditor = AsyncAceEditor(['mode/markdown', 'theme/github']);
 
 export const TextAreaEditor = AsyncAceEditor([
   'mode/markdown',

superset-frontend/src/features/home/RightMenu.tsx

@@ -571,12 +571,7 @@ const RightMenu = ({
     if (!navbarRight.user_is_anonymous && showActionDropdown) {
       items.push({
         key: 'new-dropdown',
-        label: (
-          <Icons.PlusOutlined
-            iconColor={theme.colorPrimary}
-            data-test="new-dropdown-icon"
-          />
-        ),
+        label: <Icons.PlusOutlined data-test="new-dropdown-icon" />,
         className: 'submenu-with-caret',
         icon: <Icons.CaretDownOutlined iconSize="xs" />,
         children: buildNewDropdownItems(),