title: Changing SliverAppBar title color in Flutter application
body: <p>I'm using a SliverAppBar including a background Image and Title.
  Title text is white and I need to change the color to black on the AppBar is 'reduced' (since tabbar is white as well).</p>
  
  <p>How to do that ?</p>
  
  <pre><code>NestedScrollView(
            headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {;
              return &lt;Widget&gt;[
                SliverAppBar(
                  expandedHeight: 200.0,
                  floating: false,
                  pinned: true,
                  backgroundColor: Colors.white,
                  flexibleSpace: FlexibleSpaceBar(
                      centerTitle: true,
                      title: Text(_event.name,
                          style: TextStyle(
                            color: Colors.white,
                            fontSize: 16.0,
                          )),
                      background: CachedNetworkImage(
                        imageUrl: _event?.imageMediumUrl ??
                            'http://preprod.tibib-live.com/medias/cached-media/medium/5bea5964109aa-c827b05facc3781485e584dac2f4dddc.png',
                        fit: BoxFit.cover,
                      )),
                ),
                SliverPersistentHeader(
                  delegate: _SliverAppBarDelegate(
                    TabBar(
                      labelColor: Colors.white,
                      indicatorColor: Colors.red,
                      unselectedLabelColor: Colors.grey,
                      tabs: [
                        Tab(icon: Icon(Icons.info), text: "Info"),
                        Tab(icon: Icon(Icons.people), text: "Courses"),
                      ],
                    ),
                  ),
                  pinned: true,
                ),
              ];
            },
            body: TabBarView(
              children: &lt;Widget&gt;[_buildInfo(), _buildTrials()],
            ),
          ),
  </code></pre>