Fixes #17584
Closes #17607 by merging it

Strip out .err from our syntax highlighting styles because it fucks up our placeholders in code snippets

flip things around again, no more base class, try out some new stuff to keep responsive flex grid working

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

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Refs #18782
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