fix: correct light/dark mode styles for copy button in github-auto.min.css

[VoidYun]

追加了下方代码

.hljs-copy-button {
    --hljs-theme-background: var(--bc-hljs);
    --hljs-theme-color: var(--color-hljs);
}

Fixes #36

修复 github-auto.min.css 在明暗切换下复制按钮样式错误的问题

[ruibaby]

/lgtm

[f2c-ci-robot]

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