Revert "Change documentation background color"

[howsiyu]

This reverts commit 6963ecf.

With the background-color added in custom.css the documentation page in dark mode is displayed like this.

[codecov]

Codecov Report

Merging #527 (940b1db) into main (39760ef) will not change coverage.
The diff coverage is n/a.

❗ Current head 940b1db differs from pull request most recent head ecaf0b5. Consider uploading reports for the commit ecaf0b5 to get more accurate results

@@           Coverage Diff           @@
##             main     #527   +/-   ##
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  Coverage   99.28%   99.28%           
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  Files          47       47           
  Lines        1948     1948           
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  Hits         1934     1934           
  Misses         14       14           

[howsiyu]

I am guessing what was intended is something like

html[data-theme=light] {
    --pst-color-background: #fefefa;
    --pst-color-surface: #f7f7f3;
}