fix wrong ordering of highlighting stylesheets for dark mode, and also nojs highlight bleeding #12523
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Currently we have a very bad bug with syntax highlighting in dark mode
This is because we no longer disable the light highlight stylesheet due to honoring the dark = light + dark convention.
The last commit in this PR fixes this by following the light, dark, light stylesheet pattern we are following for the main stylesheet.
I prefer this because it is consistent with our old behavior (except in nojs) and the other stylesheet. However, if others prefer a simpler approach, we could handle this case differently and disable the light highlight stylesheet instead.
The first commit is the tests, which fail on their own.
The second commit is the fixes to the highlighting styles, so that dark + light does not have the artifacts we saw for arrow, a11y
fixes #12399
fixes #12522