docs: fix unsupported nested css

[Shinigami92]

Before this fix:

⠴ building client + server bundles...warnings when minifying css:
▲ [WARNING] Transforming this CSS nesting syntax is not supported in the configured target environment ("chrome87", "edge88", "es2020", "firefox78", "safari14") [unsupported-css-nesting]

    <stdin>:4875:8:
      4875 │   .dark & {
           ╵         ^

  The nesting transform for this case must generate an ":is(...)" but the configured target environment does not support the ":is" pseudo-class.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.57%. Comparing base (4ab0731) to head (f59ff15).

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #2696   +/-   ##
=======================================
  Coverage   99.57%   99.57%           
=======================================
  Files        2846     2846           
  Lines      249917   249917           
  Branches     1056     1063    +7     
=======================================
+ Hits       248851   248859    +8     
- Misses       1037     1058   +21     
+ Partials       29        0   -29     

see 31 files with indirect coverage changes