Replace deprecated app.add_stylesheet with app.add_css_file.

[JulieSchramm]

DESCRIPTION OF CHANGES:

Replace app.add_stylesheet with app.add_csss_file for ReadTheDocs build for the develop branch. This is needed to fix:
Running Sphinx v4.1.1
loading translations [en]... done
making output directory... done

Traceback (most recent call last):
File "/home/docs/checkouts/readthedocs.org/user_builds/ufs-srweather-app/envs/latest/lib/python3.7/site-packages/sphinx/cmd/build.py", line 279, in build_main
args.tags, args.verbosity, args.jobs, args.keep_going)
File "/home/docs/checkouts/readthedocs.org/user_builds/ufs-srweather-app/envs/latest/lib/python3.7/site-packages/sphinx/application.py", line 251, in init
self.config.setup(self)
File "/home/docs/checkouts/readthedocs.org/user_builds/ufs-srweather-app/checkouts/latest/docs/UsersGuide/source/conf.py", line 113, in setup
app.add_stylesheet('custom.css') # may also be an URL
AttributeError: 'Sphinx' object has no attribute 'add_stylesheet'

Exception occurred:
File "/home/docs/checkouts/readthedocs.org/user_builds/ufs-srweather-app/checkouts/latest/docs/UsersGuide/source/conf.py", line 113, in setup
app.add_stylesheet('custom.css') # may also be an URL
AttributeError: 'Sphinx' object has no attribute 'add_stylesheet'
The full traceback has been saved in /tmp/sphinx-err-5n19bai9.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at https://github.com/sphinx-doc/sphinx/issues. Thanks!

TESTS CONDUCTED:

HTML builds on MacOS

DEPENDENCIES:

None

DOCUMENTATION:

This fixes the documentation

[gsketefian]

@JulieSchramm Can you say in the PR why this change was needed? Did app.add_stylesheet change to app.add_csss_file in a recent version of ReadTheDocs? Thanks.