Add unknown placeholder for CoreSystem

[Kyle-Ye]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 22.22222% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 24.53%. Comparing base (71adeba) to head (831f138).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/OpenSwiftUICore/Graphic/Color/SystemColors.swift	0.00%	13 Missing ⚠️
...rces/OpenSwiftUICore/Graphic/Color/CoreColor.swift	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #514      +/-   ##
==========================================
+ Coverage   24.49%   24.53%   +0.03%     
==========================================
  Files         486      486              
  Lines       28196    28199       +3     
==========================================
+ Hits         6906     6918      +12     
+ Misses      21290    21281       -9     

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