Clean up theming docs

[shilman]

Issue: N/A

What I did

Tighten up the intro to theming

[vercel]

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[codecov]

Codecov Report

Merging #6494 into next will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             next    #6494   +/-   ##
=======================================
  Coverage   40.81%   40.81%           
=======================================
  Files         616      616           
  Lines        8518     8518           
  Branches      535      535           
=======================================
  Hits         3477     3477           
  Misses       4949     4949           
  Partials       92       92

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[codecov]

Codecov Report

Merging #6494 into next will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            next   #6494   +/-   ##
=====================================
  Coverage   40.1%   40.1%           
=====================================
  Files        633     633           
  Lines       8659    8659           
  Branches     619     619           
=====================================
  Hits        3473    3473           
  Misses      5097    5097           
  Partials      89      89

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7a15a14...504cb26. Read the comment docs.