feat(theme:SettingsService): support generic

[cipchk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1207 (23bff31) into master (7ca7458) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1207   +/-   ##
=======================================
  Coverage   96.33%   96.33%           
=======================================
  Files         268      268           
  Lines        8072     8072           
  Branches     1478     1478           
=======================================
  Hits         7776     7776           
  Misses        135      135           
  Partials      161      161           

Impacted Files	Coverage Δ
...es/theme/src/services/settings/settings.service.ts	92.00% <100.00%> (ø)

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[ng-alain-bot]

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