fix base.less @at-root syntax error

[jojoLockLock]

@at-root is not a right syntax in Less.
this error will make CSS parser doesn't work

[codecov]

Codecov Report

Merging #9832 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #9832   +/-   ##
=======================================
  Coverage   86.33%   86.33%           
=======================================
  Files         197      197           
  Lines        4713     4713           
  Branches     1315     1315           
=======================================
  Hits         4069     4069           
  Misses        641      641           
  Partials        3        3

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