Customize the Editor styles based on template part being edited?

[MadtownLems]

The situation:
We have a hybrid theme (uses the Block Editor for content, but only uses Template Parts - not FSE). We are trying to get away from "widgets" and move to Template Parts instead. We'd like to replace our existing widget areas with template parts.

The problem:
Each of 2 widget areas has a unique problem when simply using a Template Part in place of the old widget area.

1. The Footer template part is displayed over a black background, whereas content is displayed over a white background. That makes it very difficult for site editors to know how their changes will look on the front end.

2. The Sidebar template part is much more narrow than the standard content areas. Again, that makes it very difficult for site editors to know how their changes will look on the front end.

Where/how can we modify the editor experience's styles only for a specific template part? For example, we want the editor canvas to be much more narrow when editing the 'sidebar' template part. We want the editor canvas to have a black background when editing the 'footer' template part.