FAL-242 theming support

FAL-242

This makes some style tweaks to improve the general layout and add theming support.

Test instructions:

• view the frontend page and verify that the default styling and layout is reasonable (this can be overridden by theming, so it doesn't need to match the client requests exactly)
• verify that the header and footer logos match the LOGO_URL and LOGO_TRADEMARK_URL env vars (see the .env files). You can modify those urls and restart npm start to verify.

This can also be tested on the sandbox: https://mfe.cloudera-search-sandbox2.opencraft.hosting/search-courses

Reviewers:

• @tartansandal