Retire light-theme footer in Firefox pages

[reemhamz]

One-line summary

We're moving a lot of our content and persona to be within a pan-Mozilla umbrella, and that includes making sure we have a consistent theme within our website, as well.

We typically had a light-theme footer placed in Firefox pages, and we're going to be switching it to the typical dark-theme footer we have across the rest of the site.

[!NOTE] We will not be removing the light-theme footer in Whats New pages.

Significant changes and points to review

There wasn't much that needed changing except only allowing the light-theme to be used in the Whats New page, so I removed the theme_class from the base-protocol.html file and instead added it to the /whatsnew/base.html file.

Issue / Bugzilla link

https://github.com/mozilla/bedrock/issues/14251

Testing

• [ ] http://localhost:8000/en-US/firefox/ (should now be dark-theme)
• [ ] http://localhost:8000/en-US/firefox/whatsnew/ (light-theme should still be active)

[reemhamz]

Just waiting on a couple of questions to be answered and then I'll mark it as ready for review

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.57%. Comparing base (3937f5c) to head (226edab). Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14307      +/-   ##
==========================================
- Coverage   76.57%   76.57%   -0.01%     
==========================================
  Files         144      144              
  Lines        7800     7804       +4     
==========================================
+ Hits         5973     5976       +3     
- Misses       1827     1828       +1     

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