ci: change theme-preview action run condition

[rickstaa]

This PR makes sure that the theme-preview action only runs when a theme was added or changed.

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Name	Status	Preview	Updated
github-readme-stats	✅ Ready (Inspect)	Visit Preview	Sep 17, 2022 at 11:41AM (UTC)

[codecov[bot]]

Codecov Report

Base: 95.20% // Head: 95.20% // No change to project coverage :thumbsup:

Coverage data is based on head (9856d45) compared to base (f9b9015). Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1903   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files          22       22           
  Lines         792      792           
  Branches      219      219           
=======================================
  Hits          754      754           
  Misses         33       33           
  Partials        5        5           

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