This updates CodeMirror and textareas inside of the Text Editor

component to work in both light and dark modes.
Light mode appears the same as before, but uses Ink's CSS properties for

inputs as a base, and uses a custom selection color of light blue.
Dark mode uses a dark cool grey background, white text, and a dark

violet selection color.
These colors may be tweaked over time in Ink.
The formatting toolbar has also been tweaked to be less noisy on dark

mode. We now use a simple, slight background instead of the Primary

button color.
We have a number of places where we try to set input and CodeMirror

styles, but these really should be considered legacy. For now, these

are only being set within the rb-c-text-editor component. Any

site-wide defaults for input styling will be phased out in favor of a

the "auto" theme in Ink, as those get added there.

Tested that text, cursors, and selections are visible with good contrast

in light and dark modes.
There's still work to be done for syntax highlighting.