Codemirror related accessibility issues

Discussion on -dev: https://groups.google.com/g/pretext-dev/c/rP5JQcQH7_8

While trying to resolve the remaining accessibility errors in my book, I ran across a few that PreTeXt and Runestone should be handling but aren't.

In active code (and other code-highlighted contexts), the css appears to be using css from "codemirror-dark.less" (line 8) even when not in dark mode. As such, the blue color for keywords does not have a strong enough contrast to pass tests. See for example https://discrete.openmathbooks.org/dmoi4/sec_seq_intro.html#subsec-seq-python

Again in active code, and probably elsewhere, we set a language attribute in a div as lang="python". WAVE complains that the language is invalid. It appears that changing the attribute to language="python" works here. But of course this needs to be set up to work with Runestone correctly.

The slider in active code contains an empty link: <a class="ui-slider-handle ui-state-default ui-corner-all" href="#" style="left: 0%;"></a> which causes an error. Why should this be a link?

The textarea element of the active code is being flagged for missing a form label. This can probably be fixed with some aria-label element? The html it issue is:
<textarea autocorrect="off" autocapitalize="off" spellcheck="false" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; min-height: 1em;" tabindex="0"></textarea>

Note that in the related issue #656, there was an error with a <label> element wrapping the free response textbox. Removing this made everything okay, but there is aria-label="textarea" as an attribute in the <textarea> element. So I think that's what we need here.

Lest you think I'm just picking on active code, I get the same missing label error for sage cells.

Here is the reference from WAVE for the missing label issue:

How to Fix It
If a text label for a form control is visible, use the element to associate it with its respective form control. If there is no visible label, either provide an associated label, add a descriptive title attribute to the form control, or reference the label(s) using aria-labelledby. Labels are not required for image, submit, reset, button, or hidden form controls.
The Algorithm... in English
An (except types of image, submit, reset, button, or hidden), , or <textarea> does not have a properly associated label. A properly associated label is: a non-hidden element with a for attribute value that is equal to the id of a unique form control a element that surrounds the form control, does not surround any other form controls, and does not reference another element with its for attribute a non-empty title attribute, or a non-empty aria-labelledby attribute

The issue names no repository files or tests; start by locating the active-code and Sage HTML generation described in the report and reproduce the WAVE findings. Done means addressing the listed contrast, invalid language attribute, empty slider link, and missing textarea-label issues without introducing the related label problem from #656.