fix(module:theme): fix keeping show scroll bar in firefox

[cipchk]

• close firefox 浏览器有兼容问题，导航菜单一直显示滚动条 ng-alain#837

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?

[x] Bugfix
[ ] 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-io]

Codecov Report

Merging #336 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #336   +/-   ##
=======================================
  Coverage   94.34%   94.34%           
=======================================
  Files         227      227           
  Lines        6102     6102           
  Branches      980      980           
=======================================
  Hits         5757     5757           
  Misses        265      265           
  Partials       80       80

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