AIRFLOW-1445: changing HivePartitionSensor UI color to lighter shade

[Acehaidrey]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Airflow-1445 issues and references them in the PR title.

Description

• Here are some details about my PR, including screenshots of any UI changes:

My PR is simply to improve the readability of the text using the HivePartitionSensor. The screen shots below show the before and after. The darker shade (nearly black) is the before, and the purple color is the after.
After -- First suggestion (not being done but second suggestion is)

After -- Second suggestion

Before

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  There is no need for testing because it's simply an aesthetic improvement and doesn't affect functionality.

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue.

[Acehaidrey]

Does anyone know what the backend tests in travis would fail for this? How can I rerun the test without making a new PR? @allisonwang I'm tagging you because I see you're on the maintainers team and have been active in PRs recently.. Hope that's okay

[codecov-io]

Codecov Report

Merging #2476 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2476   +/-   ##
======================================
  Coverage    69.5%   69.5%           
======================================
  Files         146     146           
  Lines       11391   11391           
======================================
  Hits         7917    7917           
  Misses       3474    3474

Impacted Files	Coverage Δ
airflow/operators/sensors.py	67.36% <100%> (ø)	⬆️

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[wolfier]

Yeah I've been having trouble seeing the names too due to the color. This would be a good improvement!

[allisonwang]

@Acehaidrey Wow thanks for making this change! I was also planning to fix it. The color could be even a bit lighter to make the text more clear :)

[Acehaidrey]

Hey @allisonwang thanks for the respond and glad you also noticed this :). You're right I will make it a bit lighter. I've added an image of this below too, to have a lighter shade of the indigo I had originally.

[allisonwang]

LGTM This looks much better. @aoen would you mind merging this change? thanks!

[aoen]

LGTM