[SPARK-31565][WEBUI][FOLLOWUP] Add font color setting of DAG-viz for query plan

### What changes were proposed in this pull request?

This PR adds a font color setting of DAG-viz for query plan.

### Why are the changes needed?

#28352 aimed to unify the font color of all DAG-viz in WebUI but there is one part left over.

Before this change applied, the appearance of a query plan is like as follows.
<img width="456" alt="plan-graph-fixed" src="https://user-images.githubusercontent.com/4736016/80325600-ca4d4e00-8870-11ea-945c-64971dbb752c.png">
The color of `WholeStageCodegen (1)` and its following text (`duration: total...`) is slightly darker than `SerializeFromObject`.

After this change, those color is unified as `#333333`.
<img width="450" alt="plan-graph-fixed2" src="https://user-images.githubusercontent.com/4736016/80325651-fb2d8300-8870-11ea-8ed8-178c124d224c.png">

### Does this PR introduce any user-facing change?

Slightly yes.

### How was this patch tested?

I confirmed the style of `fill` and `color` is `#333333` by debug console of Chrome.
<img width="321" alt="fill" src="https://user-images.githubusercontent.com/4736016/80325760-6c6d3600-8871-11ea-82e7-e789bf741f2a.png">
<img width="316" alt="color" src="https://user-images.githubusercontent.com/4736016/80325765-70995380-8871-11ea-8976-7020205d585c.png">

Closes #28355 from sarutak/followup-SPARK-31565.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit 7d4d05c)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: sarutak

sql/core/src/main/resources/org/apache/spark/sql/execution/ui/static/spark-sql-viz.css

@@ -38,6 +38,10 @@
   font-weight: bold;
 }
 
+#plan-viz-graph svg text {
+  fill: #333;
+}
+
 #plan-viz-graph svg path {
   stroke: #444;
   stroke-width: 1.5px;