Color of horizontal Lines incompatible with dark Mode

[Mavoort]

On a web page with white text and a dark background ("dark mode"), the pseudocode block looks a bit strange.

In particular, the horizontal lines remain black, even though the text and the vertical lines are white.

To fix this, I suggest replacing

border: 2px solid black;

inside of pseudocode.css with:

border-width: 2px;
border-style: solid;

This way you could use pseudocode.js also on a dark website.

[SaswatPadhi]

Thanks for reporting this issue, @Mavoort!

I have reviewed your changes and left some minor comments. I've also linked your PR to this bug so this issue will be closed automatically when we merge your change.