fix: Home page sponsors logos not visible

[kaushals-code]

Thanks for making a pull request to converse.js!

Before submitting your request, please make sure the following conditions are met:

• Add a changelog entry for your change in CHANGES.md
• When adding a configuration variable, please make sure to
  document it in docs/source/configuration.rst
• Please add a test for your change. Tests can be run in the commandline
  with make check or you can run them in the browser by running make serve
  and then opening http://localhost:8000/tests.html.