Style adjustments in prep for dark mode support

[chalin]

• Contributes to Support dark mode #331 by generally this PR (re-)normalizing various styles to better match Bootstrap defaults, in preparation for the support of color modes.
• Aside from alerts, the style changes aren't very noticeable, since colors are almost the same, etc.
• Reverts alert background color to match BS default
• Makes styling of code blocks uniform, whether it's highlighted or not
  • Makes copy-code button a bit more discrete
• Misc cleanup of code blocks in a UG install page

Screenshots

Before:

After: from https://deploy-preview-1908--docsydocs.netlify.app/docs/get-started/other-options/#node-get-the-latest-lts-release

[chalin]

I'm going to merge these (visually) minor changes so that I can build upon them. Comments are still welcome @LisaFC @fekete-robert @deining et al.