Make searchlite adopt host themes

[timkpaine]

Description

Make the bundled dialog inherit host-page colours and optionally take over an existing theme search field. Document the behavior and configuration.

Type of Change

• New feature
• Documentation update

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated
• Changelog / version bump (not requested)

[github-actions]

Test Results

17 tests  +4   17 ✅ +4   3s ⏱️ -1s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 41af66e. ± Comparison against base commit 892aa20.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.80%. Comparing base (892aa20) to head (41af66e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   94.28%   94.80%   +0.51%     
==========================================
  Files           3        3              
  Lines         140      154      +14     
  Branches        9       10       +1     
==========================================
+ Hits          132      146      +14     
  Misses          4        4              
  Partials        4        4              

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