[SPARK-46123][DOCS][PYTHON] Using brighter color for document title f…

…or better visibility

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

This PR proposes to adjust the font color of the titles in the PySpark documentation.

### Why are the changes needed?

For better visibility. The current title font color is not optimal for readability especially in dark mode, which can hinder user experience.

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

No API changes, but the font color for titles has been updated to a lighter shade, improving contrast and readability as below:

## Before

<img width="1025" alt="Screenshot 2023-11-27 at 5 57 42 PM" src="https://github.com/apache/spark/assets/44108233/19ee6e80-24ac-475f-94d1-1b5e95ef381a">

## After

<img width="1023" alt="Screenshot 2023-11-27 at 5 56 49 PM" src="https://github.com/apache/spark/assets/44108233/6a0d44e3-d28e-44ff-9ee2-1f5d69ef8aa1">

### How was this patch tested?

By manually building docs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#44040 from itholic/brighter_font.

Authored-by: Haejoon Lee <haejoon.lee@databricks.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

python/docs/source/_static/css/pyspark.css

@@ -24,7 +24,7 @@ body {
 }
 
 h1,h2 {
-    color:#1B5162!important;
+    color:#17A2B8!important;
 }
 
 h3 {