Update graph.css

[chenjiahui]

In some cases, the task_instancetask_instance gets a state of scheduled, so we need to provide a style for this state. Otherwise, we can't to know which node is currently in progress.

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^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, mypy and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
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• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
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  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
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[ryanahamilton]

Suggested change

	stroke: green;
	stroke: green;