Add missing drop shadow to theme previews

[willdurand]

refs mozilla/addons#15878

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Before:

After:

[willdurand]

sigh, sorry about that

[eviljeff]

you've got some stylelint errors (see CI)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.14%. Comparing base (8a24f4b) to head (911d560).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13888   +/-   ##
=======================================
  Coverage   98.14%   98.14%           
=======================================
  Files         265      265           
  Lines       10638    10638           
  Branches     3262     3262           
=======================================
  Hits        10441    10441           
  Misses        184      184           
  Partials       13       13           

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