Add dark-mode to starlette-admin

[Alwinator]

Implementation Details

• Add JavaScript section to detect system dark-mode settings and dynamically update tabler dark-mode label
• Add CSS to fix dark mode issues of select2

Fixes #378

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (2540c51) to head (5c14b23).
⚠️ Report is 58 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #733      +/-   ##
===========================================
- Coverage   100.00%   99.72%   -0.28%     
===========================================
  Files           75       86      +11     
  Lines         5817     6849    +1032     
===========================================
+ Hits          5817     6830    +1013     
- Misses           0       19      +19     

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