Entity forms using default theme font-family instead of admin theme

Problem/Motivation

When Radix theme or subtheme is the default theme, the entity forms (node/taxonomy/block) utilize Radix's font-family instead of the administration theme's font-family.

Steps to reproduce

1. Create a Radix subtheme
2. Edit subtheme/src/base/_typography.scss and add the following:

body {
    font-family: Times;
  }

3. Compile subtheme with npm run watch
4. Go to Appearance (/admin/appearance)
5. Set Radix subtheme as default theme
6. Set Administration theme as Gin or Claro
7. Check "Use the administration theme when editing or creating content"
8. Observe administration theme body font looks fine, except when viewing entity forms (e.g. /node/add/page, /node/add/article, /admin/structure/taxonomy/manage/tags/add, /block/add/content)