But the colour scheme does hijack the font weight and colour...
It doesn't "hijack" it as much as allows the user to override the font weight for specific syntax elements. If the theme doesn't make the fonts bold, they won't be bold.
...and there is no way to disable that
If you want to have "no" colour scheme, you can paradoxically [make a theme to do that](https://gist.github.com/codebrainz/fe5ae89737559c0bf92426a371bccf3b).