Highlight cells in student notebooks that require action

[taylorgibson]

Is your feature request related to a problem? Please describe.
Students occasionally miss cells that require them to enter a solution. The ... and solution prompts aren't attention grabbing.

Describe the solution you'd like
Highlight the borders of the cells in a bright color to draw attention that the cell requires a student to take action. Perhaps using metadata tags or some other way that would also allow for a screen reader and other accessibility tools to also have a cue that the cell requires student action.

Describe alternatives you've considered
n/a

Additional context
Coursekata has implemented this feature in their notebooks.