Did your editor font go default serif on WordPress 7.0?

Mine did. Like this:

I guess WordPress 7.0 assumes you are using a theme.json file these days. I’m not doing that yet on any of the sites I work on. If you want to embrace that future, you could add a theme.json file to the root of your theme, and put some typography basics only in there:

{
  "$schema": "https://schemas.wp.org/trunk/theme.json",
  "version": 2,
  "settings": {
    "appearanceTools": true,
    "layout": {
      "contentSize": "720px"
    },
    "typography": {
      "fontFamilies": [
        {
          "fontFamily": "system-ui, sans-serif",
          "name": "System Font",
          "slug": "system-font"
        }
      ]
    }
  },
  "styles": {
    "typography": {
      "fontFamily": "var(--wp--preset--font-family--system-font)",
      "lineHeight": "1.75"
    }
  }
}
Code language: JSON / JSON with Comments (json)

That will be much nicer than no styles at all.

Or if you don’t want to mess with that you can go to the upper-right three-dot menu (“Option”), choose Preferences then go to the Appearance tab and make sure Use theme styles is unchecked.

I think it’s sorta neat that you could use the same typography setup for both the front-end and back of your site this way, and if you use theme.json for the front, might as well use it for the back. But I also personally don’t mind if the back-end is entirely default styles. It’s a reminder that you’re in a CMS, and content is data, not WYSIWYG.