[Nice to have] Allow Custom CSS to be added outside of admin

[pcraig3]

Thinking this would be a an option under "Theme options" they could click and get a box to enter code into. We'd likely want some explanatory text and example, which @admoorgit can generate. This would write to the Custom HTML box in admin.

They will need this to add things like additional background colors in tables and such.

Add one more row to "NOFO theme options".

Trigger for adding this would be: if there is a complex table in the document.