fix: Add theme switcher to document view

[larseggert]

And reduce duplication in the page structure while I'm here.

Fixes #6200

[codecov]

Codecov Report

Merging #6202 (c8bd715) into main (aa955f0) will decrease coverage by 0.01%.
Report is 4 commits behind head on main.
The diff coverage is n/a.

❗ Current head c8bd715 differs from pull request most recent head d005ab3. Consider uploading reports for the commit d005ab3 to get more accurate results

@@            Coverage Diff             @@
##             main    #6202      +/-   ##
==========================================
- Coverage   88.68%   88.68%   -0.01%     
==========================================
  Files         290      290              
  Lines       40300    40300              
==========================================
- Hits        35742    35741       -1     
- Misses       4558     4559       +1     

see 3 files with indirect coverage changes