Fix API docs CSS files so dark mode works (#112023)

dev/docs/README.md

@@ -1,4 +1,4 @@
-**Welcome to the Flutter API reference documentation!**
+# Welcome to the Flutter API reference documentation!
 
 Flutter is Google's SDK for crafting beautiful, fast user experiences for
 mobile, web, and desktop from a single codebase. Flutter works with existing
@@ -8,7 +8,7 @@ and open source.
 This API reference covers all libraries that are exported by the Flutter
 SDK.
 
-### More Documentation
+## More Documentation
 
 This site hosts Flutter's API documentation. Other documentation can be found at
 the following locations:
@@ -18,9 +18,9 @@ the following locations:
 * [Codelabs](https://flutter.dev/docs/codelabs)
 * [Contributing to Flutter](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md)
 
-### Importing a Library
+## Importing a Library
 
-#### Framework Libraries
+### Framework Libraries
 
 Libraries in the "Libraries" section below (or in the left navigation) are part
 of the core Flutter framework and are imported using
@@ -31,7 +31,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 ```
 
-#### Dart Libraries
+### Dart Libraries
 
 Libraries in the "Dart" section exist in the `dart:` namespace and are imported
 using `'dart:<library>'`, like so:
@@ -43,7 +43,7 @@ import 'dart:ui';
 
 Except for `'dart:core'`, you must import a Dart library before you can use it.
 
-#### Supporting Libraries
+### Supporting Libraries
 
 Libraries in other sections are supporting libraries that ship with Flutter.
 They are organized by package and are imported using
@@ -54,7 +54,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:file/local.dart';
 ```
 
-### Packages on pub.dev
+## Packages on pub.dev
 
 Flutter has a rich ecosystem of packages that have been contributed by the
 Flutter team and the broader open source community to a central repository.